Not quite a counterfeit, not quite real
A scrapper is a pin that came off the same kind of production line as a genuine one but was never meant to reach a guest: a quality reject, an unauthorized overrun, or a piece struck from molds that should have been retired. It's a real pin in the sense that the mold was real. It's not a real pin in any sense a collector cares about.
The word has since stretched to cover outright counterfeits: pins made from scratch to imitate a desirable release, usually a limited edition with a following. For trading purposes the distinction barely matters. Both are worth a fraction of what they impersonate, and both are common enough that every long-term trader owns some without ever having done anything wrong.
Why it matters more than it used to
The pins most worth faking are the ones with the highest ceilings: small limited editions of popular characters, the pins somebody trades three trips to find. The higher the value, the more likely a convincing copy exists, so the risk scales with the size of the trade you're about to make. A scrapper of a five-dollar open edition costs you nothing. A scrapper of an LE 500 grail costs you the pin you gave up for it.