What does Pigment Dyes mean?

Available in almost any color, pigment dyes are without affinity for fibre and held to fabric via resins.  Fabric dyers who desire to create fabric that fade use them extensively.

Heddels explains Pigment Dyes

Pigment dyes are insoluble, ground pigments that only coat the outside layer of fibers instead of absorbing into them. Pigment dyes require the use of a resin or a treatment such as emulsion or dispersion to properly adhere to fibers; however, once this is done, pigment dyes are able to attach to a wide variety of fabrics.

Since the dyes only fix to the outer layer on fabrics and are thus less colorfast, they are popular with denim manufacturers for creating a worn, faded look.

Additional Resources

Jars of pigment dyes: