Omen is a threat meter. Basically, enemies in WoW decide who to attack by deciding who is the most threatening based on the abilities you use. What Omen attempts to do is provide reasonably accurate estimates of your group's relative threat level on individual enemies, so that you can see when you're in danger of pulling aggro (or, if you're next on the snack list if your tank bites it). This info is usually only critical in raids, where only tanks can survive aggro, but it's useful for any multi-player situation.