Fast credits are the credits you get when you purchase credits — via a PRO subscription, a PRO top-up, a credit pack or a gift card. Fast credits let you create in fast mode, which skips the queue. Some of the more expensive models also require fast credits.
Relax credits are the credits you earn — from your daily streak reward and badge rewards. Relax credits let you create in relax mode, where your creation is added to a queue before it’s generated.
In relax mode you can queue one creation at a time, and the “Number of images” option must be set to 1. Fast mode doesn’t have these restrictions.
For more detail, see the fast and relax mode update page.