Many answers have been offered in reply to this question and most are angling at something similar. My favorite answer is that philosophy is all of rational inquiry except for science. Perhaps you think science exhausts inquiry. About a hundred years ago, many philosophers, especially the Logical Positivists, thought there was nothing we could intelligibly inquire into except for scientific matters. But this view is probably not right. What branch of science addresses the question of whether or not science covers all of rational inquiry? If the question strikes you as puzzling, this might be because you already recognize that whether or not science can answer every question is not itself a scientific issue. Questions about the limits of human inquiry and knowledge are philosophical questions.
We can get a better understanding of philosophy by considering what sorts of things other than scientific issues humans might inquire into. Philosophical issues are as diverse and far ranging as those we find in the sciences, but a great many of them fall into one of three big topic areas, metaphysics, epistemology, and ethics.