Propulsion is the act of moving or propelling the object in forward direction. In an airplane, the engine is part of the propulsion unit along with the blades (propellers).

If you consider a jet engine of an airplane, it has blades that spin at high speeds and compress air which are then sprayed with fuel and an electric spark lights the mixture. This burning gas expands and blasts through the nozzle at the back of the engine and thus the airplane is propelled forward with high speeds. The aircraft achieves vertical motion with its aerofoil shaped wings.

In a quadrotor, we canu2019t attach jet engines, so we use motors and propellers as our propulsion unit to provide propulsion.