A light cone shows all of the locations in space that light from a point can reach as time progresses. In this demonstration, you can see how the size of a supermassive black hole affects the light cone of a point in space located roughly 3,000,000 km away (at the event horizon of a 1,000,000 solar mass black hole). As the black hole gets larger, it pulls light towards it through the effects of general relativity.