An add-on lets a tournament poker player buy an extra batch of chips on top of the starting stack that came with the original buy-in. Organisers decide when it is available: some events offer it straight away, others only once, at a fixed point such as the end of the rebuy period or a given blind level.
Two related options are easy to mix up. A rebuy is normally allowed only when a player is short-stacked or has busted, and a re-entry means registering again after elimination with a fresh stack. An add-on, by contrast, is usually open to everyone still in the event, whatever their chip count.
For tournament operators, add-ons grow the prize pool without new registrations. The price, the number of chips received and the deadline should all be shown in the lobby before players sign up.