Docs: https://github.com/hoytech/strfry/blob/master/docs/router.md

I kind of think of this like "nginx for nostr". It lets you manage many up/down/both streams to remote relays using a single process and config file, and you can edit that config file without interrupting existing streams (mostly).

You can also use nostr filters and/or plugins to filter the traffic in both up and down directions, independently.