A spline is a grooved shaft that meshes with a mating gear in order to transmit power or motion between them. In the case of a transfer case, the number of splines on the input shaft (coming from the transmission) will match up with the number of splines on the output shaft (going to the front and rear differentials). There are numerous types and sizes of splines, but for a NP231 transfer case, there are typically 30-32 splines on both the input and output shafts.