A Light Emitting Diode as an additional LED. It uses a high brightness low current LED and includes a current limiting resistor to protect the micro:bit’s output.
The black lead goes on to GND and the green lead goes on to whichever Pin 0, 1 & 2 is being used.
To turn the LED on, use the digital write block, setting it to 1. To turn off, set it to 0.