A very thin, barely-there smear of butter, margarine, or another spread on a slice of bread — just enough to say it's there.
The term is chiefly British and is tied to the phrase "bread and scrape," used from wartime rationing onward to describe a spread stretched so thin it barely counts, when butter or margarine was scarce. It's distinct from "scraps," the trimmings, peelings, and off-cuts left over from preparing an ingredient — vegetable ends, bones, crusts — that get repurposed into stocks, broths, or breadcrumbs rather than thrown away; the two words are easily confused. "Scrape" is still used loosely today, often self-deprecatingly, to describe a stingy application of butter, jam, or any spread.