My Guru St Nikka Singh gi described this secret of self-realization. it is very deep but I will said something on it. ‘Tat tam asi’ is an idea which Saints used to tell to their develped devotees . To finding the answer of’ who am I’ everybody said first ‘ I am’ in Sanskrit according to Vadantic language it is ‘ Aham asmi’ meaning ,’I am’ but it is not clear now you will put something as i am Hindu, I am Cristain.I am Scholar or something with it. But for Nirvana or Liberation this ‘ego’ should remove. when ‘ego’ starts we said it ‘Parshin Ahankara’ the effort for medition and worship or need for practice is needed to elimanate ‘ ego’ .now it is very important to understand this wherefrom ‘thought’ is arising Observer is also there. this is our self or lord.
If we understand that this is ‘ me’ then you will know the truth with fullness but it is not easy task ,beacuse we think from long time that I am body and mind. which is not true. now what happens? when we become one with mind it is called as Jiva . jiva is Birti sahit abasa . meaning when thought mix with self it is called Jiva. Some Saints said it Chtt jad ki granthi. meaning knot of Spirit and matter. when we remove this illusion by medition then we understand ‘ Tat tam asi’ meaning ‘ I am that’ for this we needed practices of worship and medition to purify our mind. this experince should be come from withen from our heart no other person can judge this only you will understand this. it is Sva samvad meaning your own experince.