How can something get smaller but retain the same amount of mass, or stuff?
It is really quite simple. If you take a sponge the size of a soda can, you can easily squish it in your hands until it is completely covered. But here is the interesting part. If you make something smaller by squishing it, its gravity becomes much stronger. Imagine then, if you squish a star into the size of an atom how powerful its gravity would become.
A black hole's gravity becomes so powerful that anything, including light that gets too close, gets pulled in. That's right, not even light can escape the grasp of a black hole.
Anatomy of a Black Hole
Black holes are made up of 3 main parts. The very outer layer of a black hole is called the Outer Event Horizon. Within the Outer Event Horizon you would still be able to escape from a black hole's gravity because the gravity is not as strong here. The middle layer of a black hole is called the Inner Event Horizon. If you didn't escape the black hole's gravity before you entered the Inner Event Horizon, then you have missed your chance to escape. The gravity in this layer is much stronger and does not let go of objects it captures. At this point you would begin to fall towards the center of the black hole. The center of a black hole is called the Singularity. This is simply a big word that means squashed up star. The Singularity is where the black hole's gravity is the strongest.
How can you fall into a black hole?
Think of the Earth. When you are in outer space you can float around. If you get too close to the Earth you will be pulled in by its gravity. On the Earth, you could leave again in a rocket ship. However, if you fall into a black hole, there would be no way to get out because the gravity is so powerful.