This refers to a trim or protective plate/garnish fitted to the upper part of the rear bumper (often the part just above the bumper face / boot lip), usually in silver (either painted, silver‐metal, stainless steel or aluminium) finish. Its purposes include:
Protection: To protect from scratches, scuffs, or damage when loading/unloading cargo (i.e. dragging items over the bumper lip).
Styling: Adds a premium accent — silver contrasts nicely with body paint to give a rugged / sophisticated look.
Branding / Trim enhancement: Sometimes comes as part of a sport or premium trim package.