Look at the circuit shown. You can recognize the circuit with light bulb and diode. The anode of the diode is connected to the positive terminal of the battery.
| 
 Circuit with a diode in forward direction. (Enlarge)  | 
      
 
 Circuit diagram with a diode in forward direction.  | 
      
  
If you connect the diode as shown, the light bulb lights up. Through the diode, a current flows. The diode is connected in the forward direction.
But what happens when you turn around the diode?