What does Sulphur mean?

Sulphur, number 16 on the periodic table, is a non-metal chemical element. Pure sulphur is use in gunpowder, matches, and fireworks.

As opposed to indigo dyeing, sulphur is a type of dyestuff used frequently to create other colours such as  black, green, and red. Although economical, it only has moderate fastness to washing and light.

Heddels explains Sulphur

In relation to fabrics, sulfur dyes are an economical choice with decent colorfastness. The fabric is placed into a bath of sodium sulfide, becoming insoluble after oxidizing with exposure to the air.

Sulphur dyes are used mainly for dyeing cellulosic fibers such as cotton or linen. They come in a wide range of hues; however, the colors are considered dull compared to other types of dye.

Additional Resources

Blue Owl Workshop x Triple Works Sulfur dyed black raw selvedge denim:

Source: styleforum.net