What does Z-Twist mean?

Compared to S-twist, Z-twist refers to a right-handed twisted yarn.

Heddels explains Z-Twist

The majority of yarns are right-hand spun, producing Z-twist yarn. Z-twist yarn is normally used to create left-handed twill, while S-twist yarn is used for right-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.

Additional Resources

Diagram comparing Z-twist and S-twist:

Raw Denim Term - Z-Twist

Source: britannica.com