Reed valves consist of thin flexible metal or fiberglass strips fixed on one end that open and close upon changing pressures across opposite sides of the valve much like heart valves do. They are intended to restrict flow to a single direction.
Reed valves are commonly used in two-stroke engines to control the fuel-air mixture that is admitted to the engine crankcase. As the piston rises in the cylinder the resulting vacuum opens the valve and admits the fuel-air mixture. As the piston descends, it raises the crankcase pressure causing the valve to close to retain the mixture and pressurize it for its eventual transfer through to the combustion chamber.
Given the fact that they operate via air pressure alone, reed valves are not as precise as rotary valves since physical inertia causes them to open later than the optimum time. Manufacturers have attempted to address this in part by creating multi-stage reeds with smaller, more responsive reeds within larger ones that provide more volume later in the cycle.
The repeated flexing of the valve material eventually causes metal versions to fatigue and fail to seat properly while fiberglass ones will merely snap off and be digested by the engine.