Selvedge (n,): or “self-edge” is the edge of woven fabric finished to prevent unraveling which is achieved through a tighter, more dense weave.
Selvedge denim refers to a unique type of selvage that is made by means of using one continuous cross-yarn (the weft), which is passed back and forth through the vertical warp beams. This is traditionally finished at both edges with a contrasting warp (most commonly red); that is why this type of denim is sometimes referred to as “red selvedge.” This method of weaving the selvage is possible only when using a shuttle loom.
Shuttle looms weave a narrower 30-inch fabric, which is on average half the width of modern shuttleless Sulzer looms. Consequently a longer piece of fabric is required to make a pair of jeans from selvedge denim (approximately three yards). – Wikipedia