Propeller speed reduction unit, basically a transmission gear reduction with no neutral or reverse. 3:2 is common.
A propeller is designed to convert rpm to air flow with the pitch determining how much air and what horse power required to turn that rpm.This information should come with the prop.A constant speed prop changes pitch to keep the RPMs constant even when throttle position is changed.
If your prop is fixed (not adjustable) you will need a governor and or rev limiter is required.
If the prop is over driven the tips will go super sonic which can cause the prop to come apart,or just be so noisey you cant stand the sound.