During a festival, what does "double growth rate for a year" imply?

In Kittens Game, the hover text (using "Theme by Kida") for festivals looks like the following:

Hold a cultural festival to make your kittens happy. (+30% to happiness and double growth rate for a year)

The wiki page doesn't explain what "double growth rate for a year" means, and it's not clear what exactly can "grow" (it's certainly not kittens).

Question: During a festival, what does "double growth rate for a year" imply?

It refers to the time span between when you build a house and the time the kittens spawn in it.

Especially during endgame scenarios where you build a lot of houses quickly, it's nice to have them fill up faster. Some metaphysics upgrades also affect this behavior.