This is YF-S201 Water Flow Measurement Sensor with 1-30Liter/min Flow Rate – Black. This sensor sits in line with your water line and contains a pinwheel sensor to measure how much liquid has moved through it. There’s an integrated magnetic hall effect sensor that outputs an electrical pulse with every revolution. The hall effect sensor is sealed from the water pipe and allows the sensor to stay safe and dry.

By counting the pulses from the output of the sensor, you can easily calculate water flow. Each pulse is approximately 2.25 millilitres.

Note: this isn’t a precision sensor, and the pulse rate does vary a bit depending on the flow rate, fluid pressure, and sensor orientation. It will need careful calibration if better than 10% precision is required. However, it’s great for basic measurement tasks!