A float valve maintains water levels by opening the valve when the ball falls below a certain angle. The ball is filled with air that will float which pushes the valve mechanism up and straight closing the valve, stopping the water and maintaining the level. An example is smaller float valves fitted to toilet cisterns to measure the flush water.

This float valve uses a floating mechanism which gets it’s force from the rising liquid which pushes the ball up and in turn closes the hole where water will otherwise flow out of.
A valve is a mechanical component that can range from a simple plate to a complex engineering design making use of many different components.
Read more about different types of valves