Frost wedging is caused by ice having a larger volume than water.
Imagine a tiny crack — in a rock or in the paint on your house. Water seeps in and when the water freezes it expands a little bit because the ice takes up a slightly larger volume than the water is was made from. So the crack becomes a tiny bit larger.
Next time a little bit more water can seep in and….
I think you will get the idea by now.
It is called wedging because a wedge also gets slightly thicker as you hammer it into a crack. Frost wedging of course doesn’t need a hammer just repetition over and over again. It will split rocks.