From Guild Wars Wiki
The skill queue is a game mechanic that keeps track of skills that you attempt to activate before completing your current action. The game displays a queued skill by using a rippling version of its skill icon in the skill bar.
- Only one skill can be queued at a time; attempting another skill will drop the first from the queue.
- Queued skills will begin activating as soon as activation completes for the first skill or after any aftercast delay ends.
- If you try to use a skill on a target that is out-of-range, the skill will queue and you will move towards the target; the skill will begin activating as soon as you are within range. However, shouts will fail if target is out-of-range.