A cooldown is the recovery time before a monster can use a special ability again. The cooldown starts the round after the ability is used. So if an ability has a 2-round cooldown, the monster uses it on round 1, then it recovers during rounds 2 and 3, and can use it again on round 4. Think of it like recharging a battery - the ability needs time to power back up before it's ready to go again.