A Presta valve is a type of valve commonly used on bicycle tires, particularly in high-performance and racing bikes. It is narrower than the more common Schrader valve and features a threaded stem that allows for precise inflation and deflation. The design helps maintain higher pressures, making it ideal for road bikes. To use a Presta valve, you first unscrew the small top cap to open the valve. Then, you can attach a compatible pump to inflate the tire. After inflating, you simply screw the cap back on to seal the valve, preventing air from escaping.