Descartes is widely considered the father of modern philosophy because he turned the focus of philosophical investigation from the world of objects, the world around us, to the thinking subject. As a renowned mathematician, Descartes wanted to build philosophical knowledge on foundations of absolute certainty, the kind one finds in mathematical theorems and equations.
That's why Descartes turned to the thinking self in his philosophical investigations. Using a novel thought experiment Descartes argued that, though it is possible to doubt the existence of the objective world around us, it is not possible to doubt our own existence. Even if the world of everyday objects in which we live was just a gigantic trick pulled on us by some evil demon, there would still need to be someone there to be tricked in the first place. Thus, so long as I am a thinking subject, I can be absolutely certain that I exist. Or, as Descartes puts it in arguably the most famous expression in Western philosophical literature:
Cogito, ergo sum. (I think, therefore I am).
Subsequent philosophers subjected Descartes' ideas to extensive criticism and revision. But to some extent, they all followed the lead of the great French thinker in starting with the thinking subject rather than the cosmos, as had been the case prior to Descartes.