I2P is a darknet similar to Tor in some ways, but it is perfectly legal and safe to use depending on what you're doing on it, of course.
It is used for regular browsing and running services on. People setup hidden services like websites or mail servers or anything on it really. Lemmy could theoretically be hosted over I2P as well, it just wouldn't federate properly with these ones on the clearnet.
Any other info you would want included? I2P can get pretty in depth but that's my basic understanding of what I use it for.
Yes. When you setup I2P, if you are on a home router with NAT, you setup port forwarding for I2P to get through. I believe it supports UPnP as well, which would circumvent this need.
You don't need to configure anything else for being connectable from the wider network. The torrent clients remain connectable allowing you to easily seed.