In World of Warcraft, pop usually means a couple of things:

1. An abbreviation for population as in server population.
2. A term generally used to describe any mob that spawns or "pop"s in the general area (usually by surprise). Also called an Add or Respawn by some people. Not a widely used term, but more accurate in some cases than "add" for the situation. It is common to prefix with re- (i.e. repop) when referring to a respawn.
3. Can mean "Use" sometimes, "Pop your cooldown/trinket/ability" = Use it