Splines are ridges or teeth[1][2][3] on a drive shaft that mesh with grooves in a mating piece and transfer torque to it, maintaining the angular correspondence between them.
For instance, a equipment mounted on a shaft might use a man spline on the shaft that planetary gearbox matches the female spline on the gear. The splines on the pictured drive shaft 
match with the feminine splines in the heart of the clutch plate, as the smooth suggestion of the axle is certainly backed in the pilot bearing in the flywheel. An alternative solution to splines is certainly a keyway and key, though splines give a longer fatigue life.[2]