A constant speed propeller is one which can change the pitch of the blades to regulate the speed of the propeller. Since a propeller is an airfoil
, just like a wing, this pitch change actually changes the angle of attack
of the propeller blades.
While the mechanism of this pitch change varies with the aircraft, most light aircraft today use a governor which controls oil pressure to a piston attached to each blade, controlling the pitch angle, and thus the RPM. The governor is adjusted by a control in the cockpit whereby the pilot adjusts for the desired RPM. Many older aircraft used a counterbalance which worked against a spring.
You can think of the constant speed propeller as similar to the transmission on a car. At low aircraft speeds, as on takeoff, when you want acceleration, a low blade angle (low pitch) is wanted along with high RPM (low gears on a car). When level in cruise flight, you then want a high pitch with a slower RPM, which improves engine efficiency and fuel economy (high gears on a car).
the constant speed propeller
page on Wikipedia.