It is a shaft which transmits the power developed by the engine to the various parts of the vehicle.In layman's term, it is a link between the input and output of the vehicle. Without crankshaft your vehicle won't even move an inch though your engine will be running.You will agree that due to combustion of fuel in the cylinder, energy is generated and due to this the piston moves downwards during the power stroke. This reciprocating motion of the piston is converted to rotary motion of the crank (Crank is the bell like structure which you can see in the image above ). With this crank, crankshaft also rotates. This crankshaft is then connected to the gearbox of a vehicle. Thus it is responsible for transmission of power.