To scrape something is to scratch or remove its surface. If you scrape your knee, you'll probably need a Band Aid.
You can scrape the toe of your favorite boots, or scrape your arm on a branch. This might cause a scrape on your skin or your shoe, a mark that shows. There is also a kind of scrape you can hear, like the scrape of a chair on a floor, and the kind of scrape, or awkward situation, you suddenly find yourself in. People also scrape things together, or gather up as much as they can find.