Imagine you have a sandbox in your backyard and you dig a big hole in it with your hands. You make the hole so deep that it reaches the bottom of the sandbox. Now imagine that instead of sand, there is hot melted rock underneath the sand.
A pit crater is a hole in the ground that happens when hot melted rock tries to come up to the surface but can't. When the rock can't come up, it will start to build pressure underneath the surface. Eventually, the pressure gets too strong and explodes, causing a big hole to form, which is the pit crater.
The hole is usually circular and may have steep sides. Sometimes, the rocks inside the hole will be very colorful and unique because they are different from the regular dirt and rocks on the Earth's surface.
So, to summarize, a pit crater is a big, circular hole in the ground that forms when hot melted rock builds up pressure and explodes from underneath the Earth's surface.