It is supreme knowledge of the east. Buddhism Hinduism and Sikhism are defining these ideas. The real part of it to define the difference between body and mind. according to Vedanta, it is a mistake that we think we are body-mind. in fact, we are Sakshi ( observer). we can watch the activities of our mind and body.
suppose my mind is talking to anyone or going in the office or market and I am just sitting in the house. but the observer in me knows everything. so the mind is separate from me. but I think that it is ‘I’ who is thinking. when pleasure and pain happen to mind ‘I’ think it is me and effects of these activities. but if I can make a gap between mind and myself which is Sakshi .then I will remain untouched to body and mind. so Vedanta says that this is an illusion to combine Sakshi and mind. you are Sakshi (observer). you never born never dies. you are that. so I am that, or Hindu saints said ‘ Ahamasmi’ I am that. by deep meditation, you can understand it. thirteen chapter of Bhagwat Gita described very briefly.