Bumpers tend to get the brunt of minor bumps and fender benders to vehicles. As a result, the bumper can get dented, broken, cracked, or scratched. Out of all these types of damage, the easiest to remove is a light scratch. I would also add that sometimes these “scratches” might actually be paint transfer, where paint has rubbed off of another vehicle onto yours. Paint transfer and light scratches are both easily removable with a machine polisher and compound/polish. If the paint from your bumper comes off, then you will need to resort to either a bumper respray or touch-up paint.
To refinish a bumper so that it looks new again, you’ll first need to repair the major damage so that it is structurally sound. And then prior to spraying primer, paint, and clear coat so that it looks pristine again, you will have to prep the bumper by filling in the damaged parts of the bumper with body filler, and sanding it smooth.