What does a single apostrophe mean in Scala?

In this slide show on ScalaActors.pdf what does the single quote indicate when the message is sent to the pong actor?

class Ping(count: int, pong: Pong) extends Actor {
def act() {
   pong ! 'Ping // what does the single quote indicate???
      receive {
         case 'Pong =>
      }
   }
}