Can anyone explain to me the concept of “I am not my thoughts”?

I am a psychology student and I have heard the phrase “I am not my thoughts” several times during my studies. But I have yet to see an actual psychological theory or a paper that would explain what this phrase actually means. I know that this concept is very common in e.g. meditation. But it doesn’t make sense to me. What is the difference between “me” and “my thoughts”? And where does one end and the other begins? I am very confused. I can’t find any academic papers that would explain this concept. I would appreciate any paper recommendations. Thank you