Ring-spun yarn is made on a ring spinning frame, an older spinning method where fibers are drawn out and twisted together continuously as the yarn is wound onto a bobbin. The process leaves small, natural variations in thickness along the length of the yarn, rather than a perfectly even strand.
Woven into denim, these irregularities show up as a subtle, slubby texture across the fabric surface, along with fades that can look slightly more textured and varied once the jeans have been worn for a while. This is one reason ring-spun denim is often associated with a more traditional, less uniform look compared with fabric made from smoother yarn.
Ring-spun yarn is more time consuming to produce than the alternative, open-end spinning, which is part of why fabric made from it tends to sit at a higher price point. See the entry on open-end yarn for how the two compare, and our denim guide for how yarn type fits into overall fabric quality.