To add a little more explanation to it, so that source doesn't render faces that aren't supposed to be rendered it "culls" them, or removes them. This can be done manually (by using the nodraw texture) or automatically when hammer runs VBSP. If the map is surrounded by a big box, it just assumes that everything within that box is supposed to be rendered.
Long story short, it means that every face, including the ones underneath the map, will be rendered in game. So as you're walking around, you're also able to in theory see the underside of your floor. This is considered bad practice.