A small motor that moves it's output arm to a position depending on the signal sent to it with a reasonable degree of accuracy & repeatability.
The brown lead with the black croc clip goes on to GND, the yellow lead goes on to the Pin 0, 1 or 2 that is being used. The red lead goes to the battery + and the battery 0V (aka GND or minus) connecting to the micro:bit GND to provide a common ground. See circuit at bottom of page.
A servo is given a pulse-width-modulated (PWM) signal that it uses to move its arm to the corresponding position from 0°to 180°. From a users point of view, you just enter the angle required in to the script block.
This servo can move small objects & operate levers but is not suitable for heavy items like windows.