(Sanskrit) This mantra is the "Shanti" (or "peace") verse with which the Ishavasya Upanishad begins. It is said to be so meaningful that all the Vedas are summed up in it.
The complete verse reads:
om purnamadah purnamidam purnat purnamudacyate
purnasya purnamadaya purnameva vashisyate.
Om shantih shantih shantih.
That is full; this also is full.
This Fullness came from that Fullness.
Though this Fullness came from that Fullness,
That Fullness remains forever full.
OM is the udgitha, the core component of all mantras in Sanskrit, giving blessing and representing the whole. The word 'purna' has many meanings, and here it suggests complete: complete without, complete within. This verse encompasses the transcendent and immanent, two very distinct streams of Indian spirituality that were integrated in the Upanishads after more than a millennium of the Vedas. The core of this is that perfection is within, not just outside oneself, and that awakening can be had on the basis of this simple realization. One of the famous 'mahavakya' statements or supreme utterances (what the guru would whisper in the student's ear as the summary of a life's study, when the student was ready) is TAT TVAM ASI or 'thou art that'. In the context of total focus on the divine, knowing its power and majesty and scope, how powerful to be told that one is that!