fTelnet speaks WebSocket, but most BBSes speak Telnet. Having them talk directly to each other would be like an English person speaking to a French person. So a proxy server can be used to act as a translator of sorts, allowing fTelnet to connect to and communicate with BBSes it otherwise wouldn't be able to.
Just select a proxy from the list below -- it doesn't have to be the one closest to you, but that'll usually give you the best performance. And you don't always have to use this proxy -- if you know the BBS you're connecting to is capable of speaking WebSocket on its own, then you can un-check the appropriate box when you add the BBS to your address book.
(proxy.ftelnet.ca hosts a real-time status page for the proxy servers, feel free to check it out and see the day/week/month uptime report for each server.)