That valve at the top of your water heater is called a pressure and temperature relief valve. Its purpose it to open when the temperature in the water heater exceeds the limit or if your water pressure coming into the water heater is set too high. If your pressure coming into the water heater is too high then you need to turn down your pressure regulator valve at the front of the house and/or check to make sure that it is still functioning properly. If your temperature setting on the water heater is at its maximum position then you may have the heat too high and may need to turn it down. The only other reason that it should leak is if the valve is defective. This valve should be tested by lifting on the tab every six months. One thing to try if you can not get it to shut off or stop dripping is to tap it lightly with a hammer. You will need to replace it if this does not stop the drip. This valve sells for about $7.00. Please consult your local plumber or e-mail for information on replacing this valve.