"I think therefore I am," or in Latin is "Cogito Ergo Sum", as far as I read about other's explanation, it talks about one's existence. In Philosophy, it is like asking "Am I something?" or "Do I really exist?"
If one conceives in his mind that he is something and that he is "man", then his conception is true and definitely, he existed.
"I think therefore I am" is another way of saying,"I think I exist."