We see that term on the labels of fine woollen clothing, and it's somehow reassuring, but most people don't know what it really means.
Worsted yarns are those made from the longer fibers of fine quality wool, which are combed to lie parallel to each other producing a smooth, clean look. The yarn is then spun with a twist of 15-25 turns per inch, five times higher than that of a regular woollen yarn. The higher the twist, the stronger the yarn.
Fabrics made with worsted yarns have a smooth finish, fine texture, an excellent drape, and wear comfortably in moderate climates.