In knitting, welts are comparable to ribs (ribbing), only they run horizontally to the knitting rather than vertically. For example, a five row welting would be formed by knitting in stockinette for five rows, and then switching to reverse stockinette for five rows, and then continuing with the desired number of repeats.
The resulting fabric has a distinctive appearance. The purled rows bulge outwards, pulling the entire knitted piece together as ribbing does, but on the horizontal. They are a little like Michelin Man tire bulges, but on a smaller scale.
Welting is used alone sparingly, often to define an area or create shape in a garment. It is more frequently seen in conjunction with other pattern stitches to create larger effects.
Source: Mary Thomas's Book of Knitting Patterns by Mary Thomas. Dover Publications, Inc. NY, 1972
With thanks to anthropod for the Michelin Man's real name!