Tatvam asi (you are that) is a sanskrit phrase that appears in all great scriptures and teachings of various gurus. What does this mysterious phrase actually mean ?
You => Yourself. This is a very simple to understand. Whatever you consider to be “you” is you.
“That” => State of perfection, enlightenment, supreme being, brahman, God etc. (It does not matter actually)
The most important and interesting part of that sentence is:
“are” : Note that here the scriptures do not say you will “become” that. The scriptures are not promising you that you will become that. It is not a conditional, it is not a possibility. IT IS A FACT.
For those who seek Moksha or enlightenment, this phrase tells you that you ARE ALREADY what you are set to seek. The problem here is not that you have to figure out a way. The only reason why you are not sure of your true nature is because of “avidya”, ignorance.
The story of Karna is a metaphor for this. Karna was the eldest of Pandavas. The first born and possibly the King of Hastinapura. He was above Dhuryodhana. However because of events beyond his control he was brought up by a charioteer. He grew up feeling inferior despite being a natural leader. He suffered immensely because of this ignorance despite being a real Prince and a King.
“You Are That” basically tells you that you do not have to worry. The true nature of consciousness is already you, it is just that you have to get rid of ignorance to understand that simple fact.