Step 1: Understand the context of the Mahāvākya.
The phrase “Tat tvam asi” is a central Upanishadic teaching that conveys the identity of the individual self with the absolute.
Step 2: Interpret “tat”.
In this phrase:
“tat” refers to the ultimate reality, Brahman or Ᾱtman (the universal Self),
“tvam” refers to the individual self.
So, the statement implies: the individual (tvam) is identical to the universal Self (tat).