As I understand it: Think of spacetime as a fabric that covers the entire universe. When the fabric is warped by objects with mass, time is warped as well. Like if you put a bowling ball on a bedsheet and it made a divot — that’s sort of analogous to how gravity works, but the key point is that warping ALSO affects time. Where gravity is stronger (eg closer to the center of the bowling ball) means time is slowed further, and infinite density at a point (a singularity in a black hole) causes a “rip” in the fabric that makes time infinite (as perceived by an outside observer) for someone who crosses the event horizon of a black hole. Nobody knows what would happen inside.

If you’re interested in this and want a book that explains it clearly, try The Order of Time by Carlo Rovelli.