When recently played cards have been transferred to a new owner or delegated to a new player, they cannot be used in battles for a period of 24 hours since they were last played.
This is an important anti-abuse measure that prevents owners of multiple accounts from farming too many rewards with the same cards.
Cards in this Cooldown Period are indicated by the Timer icon being lit up on the Collection page. When hovered over, the small pop-up will say exactly how much time remains on the timer.
Note: Cards that are in a cooldown period can not be listed for rent.