Well, here is a long explanation:
Each torrent download is divides into several pieces, all pieces are of the same size. Torrent clients download different packets from different peers, and after they download a packet, they check whether the packet has been received correctly or not. A Hash(*en.wikipedia.org/wiki/Hash_function) is calculated, and it is compared with the hash of the piece as specified in the torrent file. If they match, the piece is good, and the next piece is processed. If it doesnt match, the piece is rejected, its called a Hashfail, and the piece is downloaded again. Hashfails can occur due to various issues - loss of packets due to faulty internet connections, ratio spoof programs, or a virus-ed peer, or any reason like that. If you get lots of hashfails from the same peer, then block him.