A spline is the raised portion of the end of your axle shaft between the notched grooves cut into the axle shaft. The more splines you have, the more contact you have, thus making it stronger.
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Olram's point was that since a non-Rubicon Dana 44 rear axle uses the same length axle shaft on both the passenger and driver, it would be advantageous because you could simply carry one single spare shaft with you on the trail in the event one breaks while you're out. With the Rubicon model, the shafts are not the same length because of the carrier (or locker) used inside the differential. It's the same spline count, but different length.
Apples to apples, a Rubicon axle is superior because of the locker that comes with it, but assuming you're upgrading down the line with an aftermarket locker anyway, the equal-length axle shaft is a nice to have.