title: what is in kivy "instance" ? like on_pressed(self, instance, pos)
body: <p>I would like to know what does the word <code>instance</code> mean in kivy?</p>
  
  <pre><code>class CustomBtn(Widget):
      pressed = ListProperty([0, 0])
  
      def on_touch_down(self, touch):
           if self.collide_point(*touch.pos):
               self.pressed = touch.pos
               # we consumed the touch. return False here to propagate
               # the touch further to the children.
               return True
           return super(CustomBtn, self).on_touch_down(touch)
  
       def on_pressed(self, instance, pos):
           print ('pressed at {pos}'.format(pos=pos))
           print ('My callback is call from', instance)
  </code></pre>