A bar tack is a short, densely stitched rectangle or zigzag of thread applied at points on a garment that take repeated pulling or strain, most often the corners of pockets, the base of a fly, or belt loop ends on jeans. The tight, overlapping stitches spread the load across a small area of fabric, which prevents the seam from tearing open the way ordinary straight stitching eventually would.
Bar tacks serve a similar purpose to a metal rivet but without exposed metal, which is why they are common at points where a rivet might scratch a surface, such as some back pocket corners, or on styles designed to be gentler on furniture and car seats. Many jeans use a mix of the two, rivets at the most exposed points and bar tacks elsewhere.
The presence and neatness of bar tack stitching is a small but useful detail to check when judging how well a pair of jeans is made. See rivet for the metal alternative, and our buying guide for more construction details worth checking before purchase.