A float valve that stays out of your way and doesn't so easily break or get stuck like the common ones. Instead of the arms like used in toilets, this has a float that goes up and down in a protected cylinder. When the water rises, it automatically turns off the input line. When the water gets used and the float goes down, it turns the water back on. 

Use with a gravity rainwater system to fill a reservoir during long times without rain, in gravity backyard systems, in tent grows, and any time you need to fill a reservoir without worrying when to turn the incoming water off — this will do that for you!