It doesn’t mean anything in daily conversation. It was Descartes’s answer to the question of whether anything truly exists. Beginning with the doubt that anything at all exists, he argued that he himself must exist because he experienced an action, that is, thought, for which there must at least be either something doing it or something perceiving it.
Descartes meant that if he believes it therefore it is because once he doubts himself nothing exists but then since he experienced an action that there must be a way of perceiving himself. The ideas of the Age of Reason related to his way of thinking about life by the primary source of authority meaning that he has authority by being able to perceive something or himself.
