Waitlists occur in the system once a product has been sold out, and the waitlist function is turned on for that product. You can learn more about product settings here.
Waitlists also allow you to move interested users into available spots in the quota if another user cancels, or you increase capacity.
Users on the waitlist cannot pay for the product until it's in their cart. Users are not automatically moved from the waitlist to an available product, allowing you full control.
Find waitlisted users
Go to settings --products --> click on the status bar beside the product --> click the box at the top --> waitlisted.
Or filter for waitlisted users in user management by clicking search --> products/services --> click on the product --> waitlisted.
Remove the user from the waitlist
Click on the user's cart (the number on the left) --> remove from waitlist
Add the product to the user's cart
Click +Add products/services and selecting the product
Alternatively, you could click Increase Quota and Add to Cart to increase the product quota and add the product to the user's cart.