Shuttle looms weave a narrower bolt of fabric with a continuous, finished edge, unlike modern projectile looms that leave a loose, frayed edge. That self-finished edge is the "selvedge," usually marked with a coloured thread (often red) that shows when you cuff the jeans.
Selvedge signals traditional construction and is common on premium and Japanese denim, but it doesn't automatically mean raw or heavyweight — those are separate qualities.