Imagine you are playing outside and you throw a rock at the ground. When the rock hits, it makes a little hole, right? The Moon is like that too! Sometimes, big rocks or pieces of space stuff called meteoroids zoom through space and crash into the Moon. When they hit the Moon really hard, they make big holes called craters.
The Moon doesn't have trees or rivers to cover these holes, so we can see them all. Some are small, like a little bump, and some are really big, like a giant bowl! So, craters are just little or big dents made by space rocks hitting the Moon!