Cooldown
Some skills have a cooldown. This is a timer which prevents the skill from being immediately reactivated. If a skill has a 5 second cooldown, then after it is used, it cannot be used again for 5 seconds afterwards.
The length of cooldowns can be altered, usually by the cooldown recovery rate stat. This stat effectively divides the length of a cooldown, so an increase in recovery rate will make the cooldown smaller.
By default, cooldowns begin from when you use a skill. However, some skills will specify that the cooldown does not begin until some condition is met, such as the skill's duration expiring.