Brush clipping is a way to cut brushes into whatever shape you want as long as it's a convex. The first type of brush you will probably create is a square/rectangle shape. There is only so many things you can do with a square brush before you will want to cut it up into more interesting shapes. This is where the Clipper tool comes in. Note: "clipping" term may also refer to "usage of collision-based brushes" (bot clip, player clip, weapon clip): see Mapping manual/Bot play for more infos about that.