"Show Stopper" is a slang term for any Win Condition that ends the game immediately when it is met and is not one of the "ordinary" factional Win Conditions like the one that Town or Mafia usually has.
For instance, early Jester-based games would end when the Jester won, even though there may have been ten other players alive. However, this always felt hollow, so many individual Win Conditions are not Show Stoppers now. Now, if an individual player (such as Jester or Lyncher) wins on their own, they are simply acknowledged to have won and leave the game. They share their victory with whichever faction wins the main game.
However, some third party roles have Win Conditions that are so difficult to achieve that if they do so, the moderator will end the game with them as a sole winner regardless of the state of the rest of the game. This is a Show Stopper.