Abrasion
Abrasion is when a rough surface scrapes and wears away a softer one through rubbing.
Think of it this way
Grating a block of hard cheese against a metal grater is a good model. The rough metal surface tears away tiny bits of cheese with each pass. The cheese block gets smaller and smaller as material scrapes off. The same thing happens in nature when wind-blown sand rubs against a rock face.
Explaining abrasion by grade level
When you rub two things, the rough one scrapes the soft one. Baking soda feels gritty on a stain. The tiny grains scrub the stain off by rubbing it away. That is how a paste can clean a dirty spot.