Adjective; Someone who crouches behind a player and follows them around, usually using them as cover and/or to cause great annoyance.
Particularly prevalent on online FPS games, such as Call of Duty.
The verb is 'to bum crawl'.
Player #1: "Dude, look at that n00b behind you, he's been following you all game."
Player #2: "Yeah, he's such a bum crawler!"