AP is an acronym for Armor Points. In-game, its represented by the Green icons, like the Green Bar under the Hitpoints Bar on your HUD or like the Green Shield Icon that appears on every map in PvP or as a drop in PvM.
Armor Points, like Health, take less damage when the player is struck in certain areas of the body owing to the Hitbox system. AP can be increased by wearing armor and rings that contribute to it and it can be regenerated by either using repair kits, eating some event food or picking up the respawn green shield on any PvP map. Also, the amount of damage done to AP is affected by the weapon used to damage the player. Critical hits completely bypass AP to deal damage directly to HP, and certain attacks ignore your HP and just deal damage to your AP, for example:
-The poison element on a sword(Giston) attacks through armour and straight to the HP
-And The fire element on a sword(canopus) attacks Armour Points before dealing any damage to Health.