TAT TVAM ASI / 'That you are'
TAT* TVAM ASI is one of the great Mahavakyas or great statements in the Upanishads, which have a profound significance as pointers to Reality. It is used for advaita meditation without form.
Note: The word 'Tat’* translated as 'That' is referred to Brahman, the ultimate absolute reality which the essence and the source of all universe.
'Tat Tvam Asi’ means ‘That thou art’ or 'That you are'. or 'That I am'.
Attention! Tat Tvam Asi’ doesn't mean 'you are That' but 'That you are'!
What is the difference? You, the small egocentric personality, you are not the Brahman, but Brahman is what you really are. Brahman is what you really are and this person that you think you are is a manifestation of Brahman.
Thus if you want to meditate on this Mahavakya don’t make the mistake to consider yourself (the egocentric body-mind entity) as Brahman. You have to discard this small self and identify with your essence, the ever present silence within which is eternal existence, pure consciousness or awareness and bliss.
Everything you perceive you are not! This includes your body, vital energy, emotions, feelings, sensations, thoughts, perceptions, imaginations. What you are is not perceivable, it is pure subjectivity. You only can Be this, you cannot perceive it.
What you think you are which appears to be a subject is also an object in consciousness or Brahman. Your ego, the ‘Iness or this sense ‘I am the body, ‘I perceive, ‘I see, ‘I hear, ‘I feel etc. which appears to be a subject perceiving the objects through senses it is also an object in consciousness.
Your body is an object, your energy is an object, your mind is an object, your intellect is an object and your egoity or the sense I am the body, or I am a person is an object. What you are is the permanent substratum of all these appearanses, the eternal subject that all these appear. This substratum is infinite, eternal, unchangeable, ever present, self-existent, self-luminous, ever peaceful and blissful and is what the Upanishads call as Atman or Brahman, and religions as Shiva, God, Spirit, Allah, Jehovah etc.
Realize this and live the peace, bliss and freedom of your eternal divine nature!