A waiting list is the opportunity to redeem a ticket put back on sale by another Shotguner for an event that is sold out.
Waiting lists can be found on the Shotgun app, on the event page. At the bottom of the page of an event that is sold out, there is a green button that invites you to "Join the waiting list". By clicking, when a ticket becomes available, you are warned by email and have 10 minutes to buy it. If your card is registered and you signed up for an automatic purchase, the next available ticket will be purchased automatically and you will receive a confirmation email.
Keep in mind that you are not guaranteed to find a place by signing up on the waiting list, it depends on the number of people reselling their tickets, and the number of people in line on the waiting list.