Reed Function and Design

Two questions. Exactly what does a reed do, how does it work. And if I want to cut my own from a floppy disk, what measurement is critical, Length, width, does it have to be exactly the same size and shape as a new one or can it be longer?

Reed is a valve for fuel and air mixture which enters into crankcase. Length, width and thickness are important.

Here is thread: https://www.coxengineforum.com/t3064-how-to-make-reed-valves-out-of-a-floppy-disk