See the pieces of heavy thread laying over each boot? The thread used to sew on the welt is tapered at each end so it can be attached to a bristle, and it’s waxed with a sticky wax first so the bristle will stay on and also so the stitches will lock tightly. Once the tapers are attached to the bristles, the thread is ready to use and it’s called a “waxed end.” 
I don’t like making waxed ends because it makes my fingers sticky. But I do enjoy inseaming (sewing on the welt) so I feel happy when the waxed ends are done!