If someone hits you hard with a thin stick, your skin might rise up in a welt. Ouch. A welt is a swollen bruise.
The extra bit of stitching around the pocket of your jeans can also be called a welt as can the stitching that holds your shoe to its sole. In these cases as well as the mark on the skin, a welt is a rising where things meet –– either where a blow meets the body, or where two types of fabric meet each other.