what abrasive is ?

An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away. While finishing a material often means polishing it to gain a smooth, reflective surface it can also involve roughening as in satin, matte or beaded finishes.
Abrasives are extremely commonplace and are used very extensively in a wide variety of industrial, domestic, and technological applications. This gives rise to a large variation in the physical and chemical composition of abrasives as well as the shape of the abrasive. Common uses for abrasives include grinding, polishing, buffing. honing, cutting, drilling, sharpening ,lapping and sanding.
the information from : wikipedia 

Abrasives is called the indutrical tooth. This is because the majority of product pieces needs it to cutting ,sanding or polishing.