A spline is a ridge or tooth[1][2][3] on a drive shaft that matches with a groove in a mating piece and transfers torque to it, maintaining the angular correspondence between them.
Adjacent images in the section below show a transmission input shaft with male splines and a clutch plate with mating female splines in the center hub, where the smooth tip of the axle would be supported in a pilot bearing in the flywheel (not pictured).
The crank arm to BB shaft interfaces that are splined include ISIS Drive, Truvativ GXP and Howitzer, Shimano's Octalink and many others, most of which are proprietary.
Cassettes engage the freehub via a spline that has one groove wider than the others to enforce a fixed orientation.
Also, if the part is small it may be difficult to fit a milling or grinding tool into the area where the splines are machined.