It's a set of computer protocols that allows bulletin board users to share BBS messages and e-mails between bulletin boards. Pre-Internet. Relied upon phone calls between bulletin boards.
The weakness is that, compared to what we have now, it is s--l--o--w. The strength is that it doesn't depend upon Internet connectivity, which could eventually be filtered. It depends upon phone connectivity. With laptop computers being powerful enough to host bulletin boards, one could conceivably take a laptop and physically go to a truck stop and make the connections needed to transfer e-mails and bulletin board messages. Thus, it theoretically can be made almost completely secure and anonymous.
Hmmmm, Fidonet over Internet? If Internet is available, instantly make encrypted connections to Fidonet servers for the data shipments. If not, wait for telephone connectivity. Users would only notice loss of the Internet when things get really slow.
I was an avid user of Fidonet way back when. I’d log on and get my messages, read and answer everything, and log on and send the replies (to save on phone bills, you didn’t have to be on the whole time, and receive and send took only a minute or so). It works, you just have to keep the conversations in your head for a day or two.