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 equipment mounted on a shaft might use a male spline on the shaft that matches the female spline on the gear. The splines on the pictured drive shaft match with the female splines in the center of the clutch plate, as the smooth tip of the axle is definitely supported in the pilot bearing in the flywheel. An alternative solution to splines is definitely a gear rack keyway and key, though splines provide a longer fatigue life.[2]