The Supreme Word (Mahavakya) or divine mantra "Tat tvam asi" (That thou art; you are That) has been used as a mantra, japa and sadhana since time immemorial within the Hindu traditions (Vedas, Vedanta, Yoga schools etc.)
However, as pointed out by jnani sages (such as Sri Ramana Maharshi, Sri Sadhu Om and others), it is in fact, not a word to be used as a mantra, japa or yoga practice- much to the chagrin of yoga practitioners caught on the level of dualism (i.e.. 99% of spiritual practitioners).
It is in fact an invitation to investigate and turn one's attention back onto the first person- examining exactly what is this "I"?
Since we cannot completely know "That" in whatever external form it appears (and much less so in the sense of a Divine Godhead!), we are left with only one direction- knowing oneself in the first person, which is "I". We know ourselves intimately on a certain level, and we know that we exist.
If "I am That", what is this "I" that is being pointed to in the scriptures as being Divine in nature (and equal to "That")?
On examination of the "I", the apparent self (ie. you and me as individuals) may discover much, and rather than listing some conclusions here, it would be much more worthwhile for anyone reading this- including the writer- to investigate immediately and find out by direct experience. What is this "I", where does it emanate from? What is it's flavour and feeling? Can it be held for any length of time in awareness?
As a pointer, it would be worth considering that anything observed cannot (by logic) be the observer. Hence observing the physical sense objects such as this body, normally taken as "I", leaves no choice but to accept that the body is not "I". Continued in this way, the inquiry progresses to its ultimate conclusion.
Once and when the ultimate conclusion is reached, in silent observation and direct knowing, then the Mahavakya is clearly understood.
(Gratitude to SA, RM, SO, and the SG)