How does port forwarding work
https://en.wikipedia.org/wiki/Port_forwarding
should use it for torrents
Well, if you have an Internet-accessible host that can listen on a port and another host that it can reach but isn't directly accessible to the rest of the Internet that is running a BitTorrent client, it would allow the latter to accept incoming TCP connections.
You might have that scenario on a home network with a broadband router that's Internet accessible and a connected computer with a private IP address.
IIRC, BT can operate without the ability to accept incoming connections, but if you can let it accept incoming connections, it'll let you talk with other BT clients that can't accept incoming connections, so you may get faster downloads and uploads.
tradeoffs
Depends on what else you're considering. A dedicated Internet-accessible seedbox? Not having incoming connections?