A quite fundamental definition which confuses me many times. As English is not my original language, I fail to understand some typical words in this game.

The word "cooldown", many of you mention it as a certain type of skills in discussion, I know its meaning in general but can't get what it exactly means here.
Could someone explain it in examples, in case of BLM/DRK? For the two jobs, what're the "cooldown" skills?