planetary gearbox splines are ridges or tooth[1][2][3] on a drive shaft that mesh with grooves in a mating piece and transfer torque to this, maintaining the angular correspondence between them.
For 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 female splines in the heart of the clutch plate, while the smooth suggestion of the axle is backed in the pilot bearing in the flywheel. An alternative solution to splines can be a keyway and crucial, though splines provide a longer fatigue life.[2]