Interfaces as callbacks

I'm reading through O'Reilly's Learning Java and I'm having trouble wrapping my brain around this concept. Here's the example code they give.

interface TextUpdateable {
  void doTextUpdate( String text);
}

class TickerTape implements TextUpdateable {
  public void doTextUpdate(String text) {
     System.out.println("TICKER:\n" + text + "\n");
  }
}

class TextSource {
  TextUpdateable receiver;
  TextSource(TextUpdateable r) {
     receiver = r;
  }

  public void sendText( String s) {
     receiver.doTextUpdate(s);
  }
}

I understand the basic concept of Interfaces, specifying defined behavior through a group of abstract classes.
Can someone explain to me what's going on here?