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 planetary gearbox equipment mounted on a shaft might use a male spline on the shaft that fits 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 suggestion of the axle 
is supported in the pilot bearing in the flywheel. An alternative to splines can be a keyway and crucial, though splines provide a longer fatigue life.[2]