Glitching is going to positions on the map that were not necessarily intended by the developers. Sometimes you can climb and jump to certain locations that originally weren't planned by the developers, hence the word "glitch".
There are glitches that can take you "out of the map" or "underneath the map". To get examples, youtube videos are a great place to look. Just search COD4 glitches.
Our position at FH is that glitching is allowed as long as you can be seen and killed (Elevators are NOT allowed).