A scrape, or tracker scrape, is a request sent by a BitTorrent client to a tracker. A request is sent, connection to the tracker is established, information is exchanged, then the connection is closed. The request does something like a "wipe" or a "pass" over the tracker, and then the tracker sends information back to the client. The returned information can contain such information as, whether the tracker is on- or offline, the reason it is offline, the numbers of peers and seeds (sending a list of all peers in the swarm is usually much more bandwidth consuming just than sending a scrape result), etc. (Note that some trackers don't support scrape requests, but its still possible to use the tracker as usually.) Clients with scrapesupport scrapes the tracker many times during the course of a download, to update its information about the swarm. So you can imagine that the tracker is scraped many thousands of times for that torrent alone, even if the swarm is not very big. The tracker can usually handle this number of requests. However, if there are more requests than strictly necessary, this can destabilize the tracker and put it offline.
