What does Scouring mean?
An industrial process where dirt or starch (i.e. oil, grease, sizing) is removed from fabrics.
Heddels explains Scouring
After fabric has been manufactured, it must be scoured to remove any grease, dust, and grime picked up during production. Scouring most often involves dipping fabric into baths of caustic soda ash.
This is an important step in that it ensures a fabric will fully and evenly take dye.
Visual of do-it-yourself scouring process: