A yarn swift is a piece of equipment that is used for winding a skein of yarn. You take the skein of yarn and pull it into a circular shape and place it over the swift. The swift is a little bit like an umbrella frame without the fabric. The swift rotates as you pull the yarn off to roll it into a ball. Often, a ball winder is used in connection with a swift. It's much easier to use a swift for rolling yarn into a ball, although many people, myself included put the skein on the back of a chair to wind.