Beginners question abour events

[Douglas Phillipson <dougp intermind net>]

I can't seem to find why you return TRUE, or FALSE at the end of an 
event routine like:

static gboolean
button_press_event( GtkWidget *widget, GdkEventButton *event )
{
  if (event->button == 1 && pixmap != NULL)
      draw_brush (widget, event->x, event->y);

  return TRUE;
}

What happens if you return FALSE when you should be returning TRUE?
How do you know what you should be returning?

Thanks

Doug P