What does S-Twist Yarn mean?
A left handed twisted yarn.
Also see Z-Twist.
Rawr Denim explains S-Twist Yarn
S-twist yarn is a yarn spun counter-clockwise and is normally used to create right-handed twill, while Z-twist yarn is used for left-handed twill. By opposing the direction of the yarn and the direction of the twill, the finished material is softer than fabric created with a corresponding yarn and twill weave.
Here is an illustration of S-Twist Yarn: