Four stroke valves are what allow air to flow in and out of the cylinder bore. The camshaft in a 4 stroke engine controls the timing of the air flow by the shape of the camshaft lobe. The height of the lobe dictates the lifting of the valves off their valve seats. Engines that spin high rpm’s can control valve float much easier when the valves are light. Valve float happens when the spring can no longer keep the lifter/valves in contact with the cam, power quickly drops off and the cam will be destroyed in a short amount of time.
One way to lighten the valve is to simply use a different material such as titanium, which is very strong and light but isn’t cheap. The valve stem retainers can also be titanium to lighten the overall valvetrain weight up. The valve stem diameter can also be minimized loosing a little weight and allowing more air to flow around it where it passes through the intake and exhaust ports. Be careful though as going too thin allows the valve to flex, break or not seat well loosing power. Another popular to increase the valve weight and airflow is to use two smaller ones as in four valve per cylinder setups. Intake valves are usually larger as more area is needed to introduce enough air into the engine. The exhaust valves can be smaller as the air is rushing out of the pressurized cylinder. The exhaust valve encounters alot of heat and must seat well to transfer heat out of itself. Some extreme temp valves are hollowed out and filled with sodium that transfers heat better into the lower cooler part of the valve. While this works the valves become too large and heavy for performance use. Many people that build street engines on a realistic budget will use stainless steel valves as they hold up very well with punishment. The angles between the valve and the seat are very critical because the air passes directly around and through them. Performance engines use multiple angles to smooth the airflow around them. Race engine also tip or canter the valves at a wider angle from the centerline of the bore to make a more direct path for the air to flow.