Your car's alternator is responsible for charging the battery and powering the electrical system while the engine is running. If it fails, your car will eventually lose power and stall. In some cases, a failing alternator can cause strange engine noises or performance issues.
Most cars will run for a short time with a failed alternator, but it's not safe to drive for long with one. If you notice any of these symptoms, have your car inspected by a mechanic as soon as possible to avoid stranded on the side of the road.