They are simply a check valve of sorts.
As the piston goes up it creates suction in the crankcase.
Atmospheric pressure pushes the reeds open and lets the fuel/air mixture into the crankcase.
When the piston comes down pressurizing the fuel/air mixture, the reeds snap shut not letting the pressurized fuel air mixture back out the inlet.
They are usually used when there is some radical inlet porting present.