A player who is camping is a player that usually does not move on a specific location of a map to achieve silent and well-prepared kills. Other reasons are to hide from the enemy.
Camping has since its existence brought endless discussions about the permission of it. Some players consider it legit, while other player even consider it cheating. Camping is allowed on any game, but a server might prevent players from doing it.
In the Zombie Escape gameplay, camping has generally always been prohibited, and players doing so will most likely get slayed or kicked. For good reasons, because it only hinders the gameplay for those who want to play the game as it has been created.
A common replacement term used in Zombie Escape is 'delaying' as with camping you effectivly delay the time left to play the map when you're not progressing.