The Single Transferable Vote (STV) is a preference voting system designed to minimize "wasted" votes and provide proportional representation.
In short, rather than choosing one candidate when you cast your votes you get to rank your favourites in order. An example is the best way to show it...
Where Shall We Go For Dinner?
Imagine you were going for dinner with friends and you could either go to Nando's, Las Iguanas, Five Guys, or Pret.
You probably have a favourite option. The one you'd vote for. But if no-one else wants to go there one you probably have a second preference, right? And there might even be one that if it came to it you just wouldn't go there?
Put the candidates you like in order of preference
If you get to the point where you'd prefer to have *none* of the remaining candidates you choose RON*
If you get the point where you don't care between the remaining candidates you can just leave them It's no different to saying,
"I'd prefer Five Guys, or Nando's if not, but I don't really fancy Pret."