Plastic is literally a commodity these days, but when its formed into spools of 3D printer filament, we don’t think so. There are many characteristics that make up “quality” filament.
One terrific resource for understanding the nature of quality is a piece from a few years ago by ProtoParadigm, a well-known supplier of good quality 3D printing filament. They go into great detail to explain - sometimes with detailed mathematical formulas - exactly how filament is extruded and how bad filament manifests printing errors.
Of course, the key parameters of quality must be the consistency of the filament’s diameter and the roundness of the filament.