Tickets sometimes, there is an early sale before the official sale opens. You usually need a code + a secure link which is sent out by the artist or the event producer. These code and link are usually sent to exclusive members of fan clubs or customers of a particular company like American Express – it varies depending on the event.
Presales don’t guarantee you to get tickets, but gives those entitled 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 result 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.