Waiting lists are a way for you to be notified if more tickets become available, however, this is not guaranteed. Some organisers will have enabled a waiting list for the event if it is sold out. By signing up, there is no certainty that you will have access to buying a ticket. If more tickets become available you will be notified amongst others who have joined the list. Purchasing is on a first come first serve basis
To sign up for a waiting list, visit the event page you're interested in attending. If the whole event is sold out the Blue Get Tickets button will now say Join Waitlist. If some ticket types are sold out, an option on the ticket selection page will be shown.