On wood arrows, it involves splicing a piece of hardwood dowel/shaft onto the front end of a softwood (usually Port Orford cedar) shaft. It increases the FOC of the arrow, though not by much, and also strengthens the shaft immediately behind the point, which is the most likely and critical breaking point.
On carbon and aluminum arrows, a short piece of larger diameter shaft is glued to the original shaft right behind the point for an external foot.
Some carbon arrows I believe are now designed with an internal footing, a gradual thickening of the shaft wall toward the front of the arrow.