After breeding, kitties must wait a certain amount of time before they can breed again. This is called cooldown.
Each kitty is born with a certain cooldown speed determined by its generation.
Every time a kitty breeds, its cooldown time increases.
This is my cat, Cookie. She is a Gen 11 and has 3 kids.
She was born with a 1-hr cooldown, but after having 3 kids, she is now in the 8-hr slot.