Portalling
This is a technique where you use structural brushes called portals to seal off a cell. As the examples illustrate, you have a room with a door (or doors and windows). The walls of the room are structural brushes, so they will be rendered if you see them in an unsealed cell. If you seal off the room with a portal brush (read the tutorials, especially the one written by AmishTurtle in the COD2 tutorials), then you "seal off" rendering to that room until you can see inside it. Basically, once your camera view sees into a cell, everything inside it gets rendered. Otherwise, you don't render what you don't see, and that reduces work on your CPU.