The Rotary Disk Valve or Rotary Disc Valve is found on two stroke engines and is a specific design for the transfer of fuel and air to the engine's cylinder for combustion.
A rotary valve is, as pointed out, a disk. This disk has a section cut from it designed to allow a specific fuel and air mixture to pass through while the engine's piston is in the down stroke.
The rotary valve is attached to the crankshaft and spins at the same speed as the crankshaft. Air and fuel leaving the carburetor pass through the intake tract and through the cut section in the disk. As the crankshaft turns to push the piston up, the disk also turns thus closing off the intake tract.
A rotary valve enables the two stroke engine's intake timing to be asymmetrical which is not possible with two stroke piston port type engines. The two stroke piston port type engine's intake timing opens and closes before and after top dead center at the same crank angle making it symmetrical whereas the rotary valve allows the opening to begin earlier and close earlier.
Rotary valve engines can be customized to deliver power over a wider RPM range or higher horsepower over a narrower RPM range than either piston port or reed valve engine though they are more mechanically complicated than either one of them.