World of Warcraft question. <3?

Unless your level 80, you don't really have to worry about caps. Like the previous poster mentioned, it is the maximum of anything in the game a player can recieve such as level, buff, armor.... There are also skills which you may have to keep an eye on. If you create another character and help him by using your main character (ie giving him money or materials for the skill) he may be capped at the skill cause he's surped his skill leveling by far.