The Third Sovereign, Guru Amardas Sahib, presents a riddle to global seekers:

On the platter are placed three things,
All-Pervasive Immortal Fare is the essence.
By eating which the mind is satiated,
the gateway of freedom is attained.
O’ Truth-Exemplars! This fare is rarely obtained,
it is obtained via Guru-granted reflection.
Why take out this riddle from inside?  
Always savor it in the heart.
Eternal-Guru presented this riddle,
the Sikhs of the Guru searched and solved it.
Nanak:  Whosoever is bestowed the understanding, understands it;
All-Pervasive is obtained via Guru-oriented earnest effort.

    Guru Granth Sahib 645

So, what are those three things?

Perhaps only 0.00001% of humans can solve this riddle. If you are wondering, that’s 750 humans out of 7.5 billion people on Earth today. It may not only tease your brain to a greater extent than ever before, but it may also woo your love to levels beyond romance. It may become enriching if you find your way to live the resolved answer. But it will require using more than your Sherlock Holmes-like logic, mystic-logic? Are you ready to give it a try?

OK, so here are a few cliff notes. Etymologically, the Guru is the imparter of wisdom, the one who takes you from “ignorance” to “enlightenment.” So, the Guru in Guru Granth Sahib’s parlance is the Perfection that enlightens and gradually becomes an institution in Sikhi. Hence, the Guru refers to:
Sabad: Infinite Wisdom became Guru Granth Sahib
Nanaks I-X: Ten founders of Sikhi from Guru Nanak Sahib to Guru Gobind Singh Sahib
Khalsa: Sovereigns en-masse became Guru Khalsa Panth

Now, what is Mudavani or Mundavani? Or, alternatively, spelled Mundawani?

In the Gurmukhi script, the difference between Mudavani and Mundavani is a tippi (consonant ‘n’ without implicit ‘a’). In the language of Guru Granth Sahib, a tippi difference doesn’t change the meaning of the word as in man v mann or Gobid v Gobind. In the Gurmukhi script, the aptest transcription of “ਵ” is “v,” though the pronunciation may vary allophonically between /ʋ/ preceding front vowels and /w/. Hence, the variations like Mundavani v Mundawani or Vah v Wah.

In Sanskrit, mudra or mudran means printing, sealing, stamping, or marking. In South Asian traditions, a mudra is an energetic seal of authenticity employed in iconography. Additionally, it referred to a food tester for kings when he used to certify or “seal” the utensils that they were safe, without poison. And the thal (platter), when presented to the kings, used to be certified good for consumption via the seal.

In Pothohari (or Pothwari) dialect of Panjabi or as a distinct language, Mudavani refers to a riddle. Pothoharis enjoy posing riddles as entertainment and as a measure of a person’s wit and intellectual capacity. It was once a widespread practice at weddings to assess the bridegroom’s intellect by posing riddles. Girlfriends of the bride would serve the food on a thal, and only after solving the riddle would the groom’s wedding party be allowed to consume what’s on the platter.  In Majha, Mundavani folk songs are also available, invoking asking or solving a riddle.

The three things are not explicitly mentioned in the aforesaid sabad (hymn) by Guru Amardas Sahib.  However, Guru Arjan Sahib helps us out in Mundavani:

On the platter are placed three things,
reflect on the Truth and the Contentment.
Ruler’s Immortal Nam is also placed on it,
it is the support of all.
If someone eats and savors it,
that one is emancipated.
This thing can’t be forsaken,
every day keep it in the heart.
Nanak: Cross the dark-ignorant world-ocean in feet-surrender
All is the Transcendent’s expanse.

    Guru Granth Sahib 1428

In Sikhi, Mundavani is an integral part of Guru Granth Sahib's scriptural canon. It is always recited at the end of any complete reading, such as the Akhand Path (incessant reading) or Sahaj Path (gradual reading).  It is also recited as part of the Rahiras (conduct or wealth for life’s journey), the daily evening prayer.  

The exegetes interpret Mundavani to be either a seal or a riddle. As a conclusion to the Guru Granth Sahib, the seal is the authentication of the text. As a riddle for life, it is to be solved by the Sikh learners by decoding the message of the Guru Granth Sahib. It is also presented as being happy upon resolving it and accepting it as the final thought.
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Guru Amardas Sahib says that three things comprise the food on a platter. This food is hidden. It is to be revealed by the Guru.

Guru Arjan Sahib also says there are three things. All interpretations to date say the three things are truth, contentment, and reflection. Then they say the fourth thing is Nam (Identification with the 1Force) which sustains all.  They all agree the thal or the platter is referring symbolically to the Guru Granth Sahib.

Is reflection one of these things? Can there be a fourth thing on this platter?

Grammatically, vicaro (popularly vicharo) is an imperative, a command. Its noun is vicar. The Guru asks the seeker to: reflect; think; contemplate; meditate.

Sat is a thing. It refers to something constant or unchanging; it is interpreted as the Truth. In Sikhi, the truth leads one to a virtuous life of devotion and ethical behavior.

Santokh is a thing. It refers to satisfaction or happiness; it is interpreted as contentment. In Sikhi, it is a cessation of more haves; it is transcending one's desires to be in harmony and acceptance.

Vicaro is to think about it!

Amrit is Immortality, beyond death!

Nam is a thing. Nam is IkOankar’s existence and identity. IkOankar is 1Force; Nam is its Identity. Nam is everything. Nam is to be realized, felt, experienced, and lived. In Sikhi, it is the culture of Oneness.

‍A riddle is presented on the platter of Infinite Wisdom, colored with love and music. Now, can you solve it? When the light of the Guru shines on one, one may solve it!

Giani Gurdit Singh, in his book entitled Mundavani, painstakingly explores in forty pages the meaning of some leading men of letters “Mundanai de Arth Bhav.” However, his focus is on where the Guru Granth Sahib ends. For our purposes, the meaning of Mundavani remains between “seal” or “riddle.” For the record, the list includes: Bhai Mani Singh, Bhai Santokh Singh, Pandit Tara Singh Narotam, Giani Gian Singh, Sobha Singh Dilvali, Sant Chanda Singh, Pryay Sadhu Sute Prakash, Pryai Shiam Singh, Pandit Hazara Singh Giani, Dr. Charan Singh, Sadhu Govind Das, Avtar Singh Vahiria, Sadhu Govind Singh Nirmal Udasi, Bhai Kahn Singh Nabha, Bhai Vir Singh, Giani Badan Singh Faridkoti, Giani Bishan Singh, Sodhi Teja Singh, Sant Giani Gurbachan Singh Bhinranvale, Sant Kirpal Singh Satovali, Sodhi Teja Singh, Principal Teja Singh, Bawa Harkrishan Singh, Dr. Ernest Trumpp, Max Arthur McAuliffe, Prof. Harbans Singh, Gurbachan Singh Talib, Dr. Gopal Singh, and Manmohan Singh.

Bhai Santokh Singh completed Gur Partap Suraj Granth in 51,829 verses in 1843; on page 430, he wrote:

Then, [the Guru] wrote all Savayye in Sri Granth Sahib.
In the end, Mundavani, as a seal, was written.
Completed the whole Bani (wisdom) whose greatness can’t be expressed.
It’s the ship to cross world-ocean; by Divine Grace, one sails on it.

The Sikh world almost always doesn’t cite a single woman of letter, so I must share Dr. Nikky Guninder Kaur Singh’s feminine perspective in The Name of My Beloved:  

My greatest challenge by far has been to reveal the aesthetic dimension of Sikh literature fully.  In the final verse of the Guru Granth (Mundavani), Guru Arjan underscores the artistic efficacy of the holy volume … the sacred verse offers the food which sustains us … But the “food” is not merely to be eaten … Not through elaborate conceptualizing, but through a full and rich relishing of the sacred poetry, the individual obtains liberation from all finite confinements … In keeping with the message of the Sikh Gurus, their poetry has to be savored. Taste is a difficult sense to transmit from one tongue to another.

Now, you might be wondering how the light can shine on me. Well, it depends on what you really want. Are you just after the truth? Is your end goal contentment?  Or do you want the Nam?

The Platter will nourish all three. But the Nam includes everything and requires coaching from the Guru. Otherwise, our truth becomes relative, and our contentment remains a moving target.

This food is for our being, our existence here and now! It is not just to be consumed. It is to be savored and tasted on our tongue, allowing palates to absorb each of its distinct flavors.

Nam is the thing, the real thing, the thing which matters most to the Guru!

The riddle is only unraveled through Guru’s Way. Once the riddle is solved personally, it is lived in each space the mortal occupies. And that is how Nam strengthens the being to swim across the dark-ignorant world-ocean filled with fear and guilt.

In South Asian traditions, there were many Satis and Santokhis – in fact, these became religious orders. Many came to see the Guru; some stayed to become Namis.  

IkOankar is the Nami; beings aspire to become Namis.
Many still come to see the Guru, and rare beings ask the Perfection to grant Nam.
Will you remember the key to seal this riddle?