I will attempt to give a pedagogical lecture for non-experts and experts on new ideas in string theory and in particular D-brane physics, from a supergravity and open string viewpoint. I will explain which corners of D-brane physics are not understood and what the recent insights are. I will end with a discussion on the potential existence of zero-dimensional holography from contemplating a stack of D-instantons and its dual matrix theory. The beginning of the lecture should contain useful and basic information about D-branes.