A small submersible pump designed for Ph neutral, ambient temperature, non-sticky liquids, ie water. It has a transistor interface with current limiting resistor to the micro:bit output and needs ~6V power to run.
The black lead goes on to GND, the green lead goes on to the Pin 0, 1 or 2 that is being used. The red lead goes to the battery +V and the battery 0V (aka GND or minus) connecting to the micro:bit GND to provide a common ground. See circuit below.
Further details can be found at http://microbit-learning.co.uk/bbc-microbit-plant-watering/
It can generate a 10cm head. Its main use would be for plant / greenhouse watering projects. This shows the pump in the bottom half of a 2 litre bottle.
Here’s the circuit layout: