The angle stop is the valve between a plumbing fixture like a faucet, dishwasher, refrigerator, washing machine, toilet and the water supply pipe. Its purpose is to allow the water to the fixture to be turned off without having to shut off the main water line to a unit. Permitting the fixture to be repaired, maintained or replaced with the water off.

Once a year, fully operate the valves to the closed and opened position once or twice. When you are done, fully open it once more before closing it down a ¼ turn. Moving these valves sometimes releases loose sand or minerals into the water line to a faucet, so remove and clean a faucet aerator as well.
If the angle stop has been ignored a long time, it may be frozen in place. If this is the case, it is recommended the valve be replaced. Trying to force the valve to work may just cause leaks to occur or handles to break off. It is a good idea to also replace the water line itself that leads from the angle stop to the fixture when an angle top is replaced if the water line is not one piece with the fixture.