A few quotes from Maharishi on the MahaVakyas in a conference on 26 June 2007

There are a few words for Vedic Science and Technology; they are called Mahavakyas, great sentences—Mahavakya, the great sentence. One of these is “Sarvam Khalv Idam Brahm", all this is Brahm, Totality, the Unified Field.

‘These are called “Mahavakyas”, great sentences of the Vedic wisdom. All of them have been located in one syllable. That is the syllable of Atma, “A”. “A” expresses Atma, and “A” is the first syllable of the Veda. The first syllable of the Veda contains within it the total Veda. “A”, Atma, “Agni, Ile Purohitam”—this is how the Veda starts. So “A” is the first syllable of Veda; “A” is the first syllable of Atma.

‘We have the seer of the Veda, Rishi Madhuchhandas, who sees Veda. “Veda” means Total Knowledge. “Total Knowledge” means the Unified Field. The “Unified Field” means “Sarvam Khalv Idam Brahm”, “Neha Nanasti Kinchan”. Both these Mahavakyas go together; both these words go together. They are Total Knowledge, and nothing is there. Total Knowledge and nothing is there. It is a big zero, the Unified Field—such a beautiful thing.

Good info also from Wikipedia, the free encyclopaedia -

The Mahavakyas (sing.: mahāvākya, महावाक्य; plural: mahāvākyāni, महावाक्यानि) are "The Great Sayings" of the Upanishads, the foundational texts of Vedanta. Though there are many Mahavakyas, four of them, one from each of the four Vedas, are often mentioned as "the Mahavakyas". The subject matter and the essence of all Upanishads being the same, all the Upanishadic Mahavakyas express this one universal message in the form of terse and concise statements. In later Sanskrit usage, however, the term mahāvākya came to mean "discourse," and specifically, discourse on a philosophically lofty topic.

The four Upanishadic statements indicate the ultimate unity of the individual (Atman) with God (Brahman). The Mahavakyas are:

prajñānam brahma - "Consciousness is Brahman" (Aitareya Upanishad 3.3 of the Rig Veda)

ayam ātmā brahma - "This Self (Atman) is Brahman" (Mandukya Upanishad 1.2 of the Atharva Veda)

tat tvam asi - "Thou art That" (Chandogya Upanishad 6.8.7 of the Sama Veda)

aham brahmāsmi - "I am Brahman" (Brihadaranyaka Upanishad 1.4.10 of the Yajur Veda)

The Kanchi Paramacharya, in referencing these four Mahavakyas, says in his book Hindu Dharma[3]:

“It is to attain this highest of states in which the individual self dissolves inseparably in Brahman that a man becomes a sannyasin after forsaking the very karma that gives him inward maturity. When he is initiated into sannyasa he is taught four mantras, the four [principal] mahavakyas.

Mahavakyas  महावाक्यानि (from http://sanskrit.safire.com/pdf/MAHAVAK.PDF)

प्रज्ञानम् ब्रह्म

prajñānam brahma 

Brahman is pure consciousness. Fully awake, self-referral dynamism (of the universe) born of the infinite organizing power of pure knowledge, the Veda—fully awake totality of the individual consciousness is Brahm, which comprehends the infinite dynamism of the universe in the infinite silence of the Self.

 (Aitareya Upanishad 3.3 of the Rig Veda)

अहम् ब्रह्मास्मि

aham brahmāsmi

I am Brahman.

I am totality.

I am singularity

I am self-referral consciousness.

(Brihadaranyaka Upanishad 1.4.10 of the Yajur Veda)

तत् त्वम् असि

tat tvam asi

Thou art that.

(Chandogya Upanishad 6.8.7 of the Sama Veda)

अयम् आत्मा ब्रह्म

ayam ātmā brahma

This Atma (individual Self) is Brahm (Universal Self).

This Self is Brahm (the wholeness of life).

This pure, silent, simple singularity of Atma is the Totality—Brahm.

(Mandukya Upanishad 1.2 of the Atharva Veda)

सर्वं खल्व् इदं ब्रह्म

sarvam khalvidam brahm

All this is Brahm—Totality.

(Chhandogya Upanishad, 3.14.1)

नेह नानाऽस्ति किं चन।

neha nānāsti kiñcana

There is nothing else anywhere.

(Brihadaranyaka Upanishad 4.4.19)

दूरे दृसं गृहपतिम् अथर्युम्

dūre dṛsaṃ gṛhapatim atharyum

Far, far away the indweller of the house, the Self, is seen reverberating.

(Rik Veda, 7.1.1) Rishi Vasishta

http://press-conference.globalgoodnews.com/archive/june/07.06.26.html