Splines are ridges or the 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 gear mounted on a shaft might use a man spline on the shaft that matches the female spline on the apparatus. The splines on the pictured drive shaft match with the female splines in the heart of the clutch plate, while the smooth suggestion of the axle is certainly backed in the 
pilot bearing in the flywheel. An alternative planetary gearbox solution to splines is definitely a keyway and essential, though splines give a longer fatigue life.[2]