You can still torrent without forwarding a port, for example by only making outgoing connections. But that may limit your speed, and figuring out forwarding should help.
Your client may have set a UPnP request that your router respects...hard to say. You didn't post any router config info, so it's hard to say what your environment looks like or was previously working. UPnP is frowned upon for security reasons, so it's best to just forward if in doubt.
It seems like that port needs to be accessible from the public Internet. Your local computer probably has at least one more firewall between it and the Internet, running on your router. You need to also forward the port on your router, which is what it says in the second half of the guide.