The tyre valve (specifically the Schrader valve) is the small stem sticking out of the wheel rim used to inflate the tire. It consists of a rubber or metal stem and an internal spring-loaded valve core that seals the air inside. When you press the center pin, air is released or allowed in. Old rubber valves can crack and leak at the base, leading to slow leaks. In modern cars, the valve stem is often part of the TPMS (Tire Pressure Monitoring System) sensor, making it a more expensive and rigid electronic component.