What is meant by Descartes phrase: "I think, therefore I am"

Descartes here is describing the inability to prove anything else but for the fact that he is thinking about proving his own existence. He states that all our physical senses, such as our visual, can be tricked or be incorrect e.g. through optical illusions. Therefore, we cannot trust our senses to deliver the truth. The only certainty Descartes proves is his ability to know for sure that he is thinking, thereby proving his existence.

Descartes's philosophy is like taking down a wall of bricks. Each brick represents an alleged statement of truth. If the statement can be falsified, then the brick gets through away. Descartes's purpose is to see how much of the wall is left untouched and still standing.