A creature's action queue stores the actions (in order) that the creature will take, barring outside influence. For player characters, this queue is displayed as a sequence of icons across the top of the screen, starting on the left side. Players can add to the queue by ordering certain actions (such as casting a spell), and they can remove actions from the queue by clicking their icons. Not all actions will add to the queue though. In particular, movement and combat clear the queue.
Furthermore, the action queue can be locked by a script (specifically by the SetCommandable() command), which will prevent any changes to the queue, aside from the removal of actions as they are completed. This is done, for example, as part of the implementation of fear.
Non-player characters also have an action queue, one that is usually controlled by scripting, but that can also be controlled by a Dungeon Master possessing the creature.