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 gear rack instance, a equipment mounted on a shaft might use a male spline on the shaft that fits the 
female spline on the apparatus. The splines on the pictured drive shaft match with the feminine splines in the center of the clutch plate, while the smooth tip of the axle can be backed in the pilot bearing in the flywheel. An alternative solution to splines is definitely a keyway and crucial, though splines provide a longer fatigue life.[2]
