When short fibers work themselves out of yarn or fabric and are subject to abrasion, they form little balls of fiber called pills. No doubt you’ve noticed them forming on your favorite hat or sweater over time. Washing clothing inside out and hanging it to dry (instead of using the drier) can reduce pilling. Pills form an essentially all fabrics, even synthetics.
As a spinner, it’s good to understand the process behind pilling because it drives home the importance of good fiber preparation. When sorting/skirting a fleece, we must be extra careful to remove the short fibers that can weaken our yarn and make it subject to pilling. Of course it’s tempting to try to keep as much of the fleece as possible, but in the end, the quality and longevity of your product will be better if you discard the short stuff.