This package contains the apt debtorrent transport. it makes it possible to use 'deb debtorrent://localhost:9988/foo distro main' type lines in your sources.list file.
for an overview of the debtorrent program, see the 'debtorrent' package.
you don't actually need this package to use the debtorrent program, it will work fine using the regular http:// transport.
however, using this method has some advantages over http. unlike the traditional http method, this transport will send all possible requests to debtorrent as soon as it recieves them, which will speed up the download as peers can be contacted in parallel. this method also allows the debtorrent client to return files to apt in any order, which is important since bittorrent downloads proceed in a random order. additionally, this method uses a very similar protocol to http, and so can easily be used to access a debtorrent client running on another host.