Rain Delay is actually a cancellation of the watering. It will prevent the timer from running any station during the delayed period. The automatic rain delay will always set the delay for 24 hours, but it will check again the next day and create the rain delay again if it is necessary. The delay will be for every program in the timer, not just a single one. They are automatically programmed, so you won’t need to do anything about them.
If you end up not wanting them to be automatic, this can be turned off by going to Settings > Devices > Your Device and turn smart watering off.