The Constant Speed Unit Propellor (CSU) endorsement will teach you all there is to know about flying an aircraft with variable pitch propellor.
A variable pitch propellor is able to change the angle at which its blades strike the air. This allows it to efficiently convert power from a larger engine into thrust and therefore allow the aircraft to be larger, heavier and faster than possible with a fixed pitch propellor. The constant speed unit is the most common way to control the pitch of the propellor.
This endorsement needs to be undertaken in order to gain access to the more advanced aircraft in the Curtis fleet that utilize a Constant Speed Unit Propellor (CSU) such as Cessna 182s, Piper Dakota, Beechcraft Baron etc.