A transistor that will allow other items to be switched where the micro:bit would not be able to. Items such as motors, larger speakers, relays and so forth, up to 1A total load. A current limiting resistor is in the circuit to protect the micro:bit output.
The black lead goes on to GND, the yellow lead goes on to the Pin 0, 1 or 2 being used and the blue lead goes to the device being switched. The "other half' of the device will need the battery + and the battery 0V (aka GND or minus) connecting to the micro:bit GND to provide a common ground.