on simple terms..Port Forwarding is a simple method of setting up a dedicated port on ur PC wich will specifically and especially used for Torrent downloads..
Although the port 80 (for http) is always open for internet access, torrent or larger file sharing requires a dedicated port so that the data doesnt conflict with the port 80 (wich as it is, is hell lotta busy with http requests and responses)..
Also, as the port becomes dedicated, d/load speeds do get better (but doesnt increase above a specific limit, set by ur ISP)..
Torrents work on the principle of sharing, its a p2p network so u will d/load sumthing and in return must giv (upload) back or share the data to the community....
if not, then such thing is known as leeching and ;eechers (more precisely, those IP addresses) are banned from the n/w and so eventually u are NOT able to d/load anything..