The phrase Tat Tsvam Asi (You are That!) is spoken (in the Chandogya Upanishad 6.1) by Uddalaka, who is teaching his precocious son, Shvetaketu, about ultlimate reality. According to Uddalaka, our perception of the plurality of objects is an illusion of speech … The truth is that just as the essence of every clay pot ~ regardless of its shape or size ~ is simply clay, so the essence of all manifest forms (including you and me) is Brahman (God, Spirit, Allah, Buddha Nature, the Tao, Whatever):

ayamAtmA brahma tattvamasi aham brahmAsmi prajnAnam brahma

Thou art that
Atman and Brahman are one
I am Brahman
All this is indeed Brahman

And what exactly is this “Brahman” which is the essence of who I am? According to the Katha Upanishad, the best (and only!) way to describe the Ultimate reality is by applying a via negativa sort of strategy, i.e. by saying what it isn’t … and hence the phrase netti, netti, “not this, not this” ~ which is to say that Brahman is without the kind of qualities we usually perceive and associate with forms in the phenomenal world:

The supreme Self is beyond name and form, Beyond the senses, inexhaustible, Without beginning, without end, Beyond time and space, and causality, Eternal, immutable.

~ Katha Upanisad 2.2.2

We find a similar teaching (that no formed or formless “object” can be considered to be the ultimate reality) in chapter 1, verse 62 of the Avadhuta Gita of Dattatreya (a wonderful Advaita Vedanta text, translated here by Swami Ashokananda):

Always “not this, not this” to both the formless and the form. Only the absolute exists, transcending difference and nondifference.