This is a rain water level detection sensor that outputs an analog voltage proportional to the water level. It can measure 0-40mm. Usage is very simple. Stand it up in the vessel you want to measure. Secure it in place. Connect - to GND, + to 3.3V or 5V, S to your Voltmeter, microprocessor or ADC to read the output voltage.
Then use the data in the calibration table to give an estimate of water level.
I calibrated one with our local tap water. Here are the results. Your voltage reading can be converted into an approximate water level reading using the table or graph below. For full accuracy, calibrate with your own water and on your own local setup...
As you can see, the results are far from linear, so, for best accuracy you would need to have some kind of lookup or mapping in your code. e.g.