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 gear mounted on a shaft might use a male spline on the shaft that fits the feminine 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 suggestion of the axle is certainly supported in the pilot bearing in the flywheel. An alternative to splines is a gear rack keyway and key, though splines give a longer fatigue life.[2]