Gravity curves the spacetime around it. We have a hard time visualizing what that looks like in three dimensions, but you can think of a massive body as bending spacetime inward toward itself. Being so massive, black holes bend spacetime to extremes. The curvature is so great that, for an object within the event horizon, there is no trajectory in spacetime that will take you outside the black hole. Rather, spacetime is so curved that all trajectories into the future converge on the singularity at the heart of the black hole.