This Thumb Joystick is similar to the 'analog' joystick on PS2 (PlayStation 2) controllers. The X and Y axes are two ~10k potentiometers which control 2D movement by generating analog signals. The joystick also has a push button that could be used for special applications.

When the module works, it outputs two analog values and a digital value. The 2 analog outputs represent the location of the two potentiometers and the digital output represents if the button has been pressed.