Organizers of events can define a maximum number of attendees. They also have the option of making a waiting list available for the event that the maximum number of attendees has been reached. You can both register for and cancel the waiting list.
By clicking on 'Register for the waiting list' you are taking a specific position on the list, depending on the number of registrations at that time. This position is visible as soon as you complete the registration. After registering, you can also cancel your place on the waiting list.
As can be seen in the first notification, you will be immediately registered as a participant in the event as soon as there is a place. Just as with a direct registration, this will be followed by a confirmation email.