csg subtract is used to remove part of a brush using another brush as a template of sorts. say you wanted a hole in your wall, make the wall then make a block of any shape and put it inside your wall, make it stick out the sides though, then hit csg subtract and then delete your block and wala! you have a hole
Thats pretty much it - but be VERY careful when using CSG Subtract - its best to use simple shapes rather than complex ones. It can lead to really frustration errors if you don't use it with care.
