Interrupt armor rating (IAR) is the number of crowd control attacks that this character is protected against. The number will regenerate over time. On NPCs with this rating, it is displayed on a red shield to the left of their health bar when targeted.
Interrupt Armor is an NPC defense type used across many MMORPGs that defends against interrupting attacks. Interrupting attacks can be used to cancel an ability that is currently being cast by the foe, such as a regeneration ability, preventing it from taking its effects. Some high level enemies intended to be fought by parties will make use of their IAR to prevent single players from beating them, for instance, by casting a regeneration ability when their health hits a certain threshold. While a single player may have interruption abilities at their disposal, many of these special enemies will have several layers of Interrupt Armor that causes them to completely drown out the effects, requiring multiple players to use one or more of their interruption abilities at the same time.