Moisture-absorbing means that the fabric can absorb sweat and such without feeling wet to the touch, trapping the moisture in tiny gaps inside the fibers. Many natural fibers such as cotton are moisture-absorbent. The upside of moisture-absorbent fibers is that sweat doesn’t leave a stain the way it might on a polyester or nylon fabric. Moisture-wicking fibers keep sweat and moisture on the surface, where it moves along the fabric’s weave, eventually reaching the outside where air can evaporate it. While this can be great in some fabrics, it’s less desirable in sheets, where the body can trap wicked moisture and keep it from evaporating.