It is a way of describing the individual unit making up a thread. In making cloth, an individual "staple" is not used, but is twisted along with other "staples" into a thread or strand, which is then used to weave or knit or crochet the cloth.
Cotton can also have a short staple, or in the finest cotton, a long staple, such as found in Pima Cotton...
Hope that helps. The reason I know this is that I am a spinner, weaver, and knitter, and this is not the only term used in desribing the individual unit that is spun to make the thread... there are other terms used, bbut do not want everyones' eyes to glaze over!