Open-end yarn, also called rotor spun yarn, is produced on machines that spin fiber inside a rotating rotor rather than winding it continuously the way a ring spinning frame does. The method is faster and less labor intensive, and it produces yarn that is more uniform in thickness along its length.
Woven into denim, open-end yarn gives a flatter, smoother fabric surface with less of the irregular texture seen in ring-spun denim. Fades on open-end denim tend to look more even and less textured, which some wearers prefer for a cleaner appearance, while others favor the more varied look of ring-spun fabric.
Because open-end spinning is quicker and cheaper to produce, it is common in mid range and mass market jeans, where cost and consistency matter more than the traditional texture that ring-spun yarn provides. Neither method is right or wrong, they simply produce a different look and feel. See our denim guide for how yarn type fits alongside weight and weave when judging fabric.