for i in range(NUM_LEDS):    # get the num range (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
        clear()              # call the clear function to turn off all leds
        np[i] = color        # set the color of first LED
        np.write()           # write the LED status
        time.sleep(delay)    # wait for delay time.
        
        # now the loop will do the same but for LED2, 3, 4 etc.
        # we clear the LEDs each time so that only 1 LED is ever illuminated.