The brush loop is a very spinny loop where you just graze the ball. Instead of getting heavy contact hitting through the ball, you just catch the outer side of the ball, creating maximum spin but with less speed.
In a loop stroke, you can brush the ball by closing your racket angle. When you brush the ball, you'll have a softer feel of the ball and the sound of the contact will be quieter, whereas a flatter hit will have a louder sound.
These are sub-pages of the Brush Loop page (a child in the hiararchy).