Tickets for some events are available to book in a presale before they’re released to the general public. You’ll usually need a password or a secure link which is sent out by the venue, artist, team, promoter or 3 Network.
Presales don’t guarantee you to tickets, but they do give you the chance to book tickets earlier, with fewer people trying to book at the same time. So if you are trying to find tickets and the results are "currently not available", that means the presale allocation has sold.
We’d never say that tickets are sold out in a presale because you’ll always have the chance to book in the general sale as well.