The word scrub has several meanings:

1. To clean something by rubbing it hard, usually with a brush and soap or water. When you scrub a dirty pot, you work at it vigorously to remove stuck-on food. Surgeons scrub their hands thoroughly before operating. A janitor might scrub floors on hands and knees with a stiff brush. The harder you scrub, the more effort you're putting into the job.
2. Low, dense vegetation made up of stunted trees and bushes, common in dry or poor soil. Scrubland stretches across parts of the American West and Australia, where tough plants survive with little water. It's the kind of landscape where mesquite and sagebrush grow thick and tangled.
3. In sports slang, a player who isn't very good or doesn't try hard. If someone calls another player a scrub, they mean that person isn't skilled or doesn't pull their weight on the team. It's not a kind thing to say, but you'll hear it on playgrounds and basketball courts.

The cleaning meaning is the oldest and most common. When your parents ask you to scrub the bathtub, they want more than a quick wipe: they expect serious effort with a sponge or brush to get it truly clean.