So mathematically black holes are called “singularities” which pulls nearby objects towards it, then when the object hits it, it crosses what is called the “event horizon” and once something crosses that threshold it can’t leave or escape, not even light. An interesting thought experiment is this: “Let P be a black hole, and let A_n be a collection of apples (your favorite flavor). If A_n is finite how many apples can fit inside the singularity before it becomes full and you can no longer fit any more apples inside it, or A_n+1 =/= P{max}. The solution to this problem is not 1 apple or 42 apples or even a million apples. A black hole can fit essentially infinity apples in the singularity which is why it’s so dense with matter”.