Good question! Think about it this way: you've always learned that what goes up, must come down, right? If you throw a baseball up from the surface of the Earth, the Earth's gravity will make it fall back down. Well, it turns out that's not completely true: if you throw a baseball away from the Earth fast enough, it will keep going forever and completely escape the pull of the Earth's gravity. The speed at which you need to throw a baseball to make it leave the surface of the Earth forever is called the "escape velocity" of the Earth (it turns out to be around seven miles per second!).

Now, if you were to make the Earth smaller but keep its mass the same, the gravitational force on the baseball at the surface of the Earth will get stronger. That means that you'd need to throw the baseball even faster to make it escape the Earth: we say that the escape velocity increases as you make the Earth smaller (but keep its mass the same).

Can we keep going? Can we make the Earth smaller and smaller, making the escape velocity bigger and bigger? Nope! Remember that nothing can go faster than the speed of light, so if we make the Earth so small that the escape velocity equals the speed of light (that's 186,000 miles per second!), then nothing can escape from it. In fact, it turns out that when you make the Earth small enough, not even light itself can escape from it surface; gravity becomes so strong that if you tried to shoot some light off of the Earth, it would just fall back down! Because nothing can get off the Earth, it would look completely black from far away, and it would look like a hole that things can fall into but nothing can get out of, so we call it a black hole. That's what a black hole is: a place where gravity is so strong, no even light can escape.

A black hole is an object that is so massive and so dense that it exerts gravity that is so powerful that nothing, not even light, can escape it.