An angle stop is the small shutoff valve where a fixture's water supply line meets the pipe coming out of the wall, found under sinks and behind toilets. It lets you shut off water to that one fixture without touching the rest of the house. Plumbing code requires a shutoff at most fixtures.
An angle stop is the little valve tucked under a sink or behind a toilet, right where the flexible supply line for the fixture meets the water pipe coming out of the wall. It gets the name "angle" because it turns the water flow 90 degrees, from a pipe running horizontally out of the wall to a supply line rising up to the fixture. Plumbers also call it a supply stop, a fixture stop, or simply a shutoff valve. They all describe the same part.
Its job is convenience and control. An angle stop lets you cut off water to one fixture without shutting down the whole house. Say a toilet's fill valve starts running and will not stop. Instead of closing the main shutoff and leaving the entire home without water, you reach behind the toilet, turn the angle stop, and the toilet alone goes dry while every other tap keeps working. That single valve is what makes almost any fixture repair, from a faucet cartridge to a toilet flapper, a quick job rather than a whole-house event.
Plumbing code treats these shutoffs as a requirement, not an upgrade. UPC 606 calls for a shutoff valve on the supply to most fixtures and appliances, and requires that those valves stay accessible so someone can actually reach and turn them. Bathtubs and showers are the usual exception, since their valves are buried in the wall. For a sink, toilet, dishwasher, or water heater, though, a reachable shutoff is expected. For example, a homeowner who finds a leaking supply line under the kitchen sink should be able to stop it right there at the angle stop, without hunting for the main valve outside. See how to shut off water to a fixture.
Angle stops come in two main styles. Older multi-turn stops use a rubber washer and take several turns of the handle to close. They are prone to seizing up or leaking after years of sitting untouched. Modern quarter-turn stops use a durable ball valve inside and close with a single 90-degree flip of the handle. They are more reliable and far less likely to fail when you finally need them, which is why most plumbers now install the quarter-turn type on any fixture they touch.