Staple yarn is yarn spun using staples, which are short fibers ranging from 2 to 8 inches. A staple---these short fibers---can be of man-made origin or of natural origin. Man-made staples are fibers which were cut short from a synthetically made continuous filament. Among natural fibers, silk might be very long, but all other natural fibers are naturally staples. To illustrate, wool shorn from a sheep will always be limited to several inches.
One detraction against a carpet milled or a rug woven from a staple yarn is shedding fiber.