The rain delay is to forbidden watering in the next n days after rain, where n is set to be 5 by default. After you set the rain delay, Netro will update the watering schedules automatically, which can be checked on the schedule page. You can also find the constraint added. If you go to the "rain delay" again, Netro assumes you want to add another "rain delay" constraint, instead of showing the last rain delays.
Please note that rain delay is just one-time setting. It's mainly used when it's raining but the weather forecast data is incorrect, so you can tell Netro that I don't want watering in the next n days. If the weather data is correct, Netro will not water while raining. You should set in settings->preferences -> rain -> days to skip, if you want to set it for all rainy days.