Combat Timer displays a timer which shows the time until you leave the PvP combat if you don't participate in it (no offensive action, heal or get attacked). The addon is made for rogues and casters to know how long until they approximately get ooc especially in arena or duels.

So whenever you enter combat the timer counts from 5 to 0. If you participate in combat the timer starts again.

Hint: The timer may be incorrect in bgs and whenever npcs enter combat (e.g. in AV). The timer will also be disabled in instances.

You can configure Combat Timer through Esc -> Interface Options -> AddOns. Though /ctimer or /combattimer is still aviable.