Fichte believed that Kant was mistaken to argue for the existence of a noumenon, of things as they are, not just as they are perceived through the categories of human reason. Fichte saw the rigorous and systematic separation of "things as they are" (the noumenon) and things "as they apear to be" as an invitation to skepticism.
Rather than invite such skepticism, Fichte made the radical suggestion that we should throw out the notion that there is a noumenal world and instead accept the fact that consciousness is not grounded in a so called "real world." In fact, Fichte is famous for originating the argument that consciousness is not grounded in anything outside of itself. This notion eventually becomes the defining characteristic of German Idealism and is thus essential to understanding the philosophy of Hegel, and Schopenhauer, though they both reject Fichte's notion that human consciousness is itself sufficient ground for experience, and therefore postulate another "absolute" consciousness.