“Bristle” can refer to any of a number of thinner alternatives to metal sewing needles used to sew inseam or outseam stitches. Historically, shoemakers used guard hairs plucked from boar pelts for this purpose, but many now also use wire or plastic.
Key advantages of bristles over needles include thinner diameter and the lack of any bulge around a needle eye. Bristles can therefore be pulled through without reaming out stitching holes, and may be easier to insert two at a time through the same hole.
The lack of an eye requires mounting thread to bristle in some other way. Tapers with bristles attached are referred to as waxed ends.