This is what happens if you do not lubricate these splines - these should be square, not triangular, and notice the chunks missing:
Because of the rotational forces within the rear drive, regular grease would be slung off the splines. Therefore, special Moly 60 paste should be used:

The ONLY grease that should be used on the spline is genuine Moly 60 paste. This is not the place to cut corners!
**NOTE: Honda has discontinued the original Moly 60 paste shown above, and replaced it with high-Moly M-77 paste:
The drive side spline has a splined gear that extends outward, and rotates as the driveshaft rotates:

The wheel side spline has a splined gear that accepts the drive side spline:

The procedure explaining the removal of the wheel and drive unit, and applying the Moly 60 paste is covered in detail in How to remove and reinstall your rear wheel. You should lubricate your splines every time your rear wheel is removed!