The waitlist is a feature in the Urban Sports Club app (doesn't work on the website), that will allow you to be placed on hold for your favourite classes (if they're already fully booked).
Every time that you make a booking through the app (instant booking), and if that class is already full, i.e., no places left, you can enter the waitlist. Once a spot becomes available, you will be notified, and you can book your spot.
You can enter how many waitlists you want, and if you change your mind you can also cancel your attendance in the waitlist.
Remember to allow Urban Sports Club notifications, to be notified when a spot becomes available.
After receiving the notification, that a spot became available, all you need to do is book the activity:
You need to be fast in booking the activity because popular courses are booked super fast, and like you, other members that are on the same waiting list will also try to book that class and can happen that the spot will no longer be available.