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 in the case of 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.