In this example, "scrape" means the tree branches are rubbing painfully against the person's skin. Here is a picture: http://img.webmd.com/dtmcms/live/webmd/consumer_assets/site_images/media/medical/hw/n5551331.jpg "Scrape" can also mean to remove something from an object. You can "scrape paint off a wall" or "scrape food off a plate." Here are some pictures: http://www.pkc.gov.uk/media/image/2/6/plate_scrape.jpg http://f.tqn.com/y/homerenovations/1/S/B/_/-/-/8.JPG

In your sentence the branches scrape. "They move in sliding contact with a rough surface" (with each other in this case as wood is rough).