In the final drive unit, the engine, through the transmission and driveshaft, drives a small pinion gear, which drives the larger ring gear, which then drives the rear wheel. The final drive ratio is the ratio of the number of teeth on the ring gear to the number of teeth on the pinion gear, and it's stamped on the final drive unit near the driveshaft oil filler hole. The ratio determines the speed reduction in the final drive unit. High ratios provide good acceleration but a lower top speed, while low ratios provide a higher top speed with reduced acceleration. The spline is composed of interlocking lands on the two shafts which are cut parallel to the shaft.
Lands are to splines what teeth are to gears.
The way the lands are cut, the two shafts can slide freely on each other longitudinally along their shafts, but rotationally they must rotate together. Radially, the two shafts will also be kept concentric by the spline. The spline on the final drive is the piece of the ring gear which engages the rear wheel, and a corresponding coupling nut which is riveted to the rear wheel hub. This construction allows the rear wheel to be removed from the final drive by simply removing the rear axle, which is important to the suspension of the motorcycle but is not involved in the transfer of power from the engine to the rear wheel. Once the wear on the lands begins, I imagine that the rate of wear accelerates.