Try starting from the relay side: "if you publish your content to a bunch of servers and people also read it from a bunch of random untrusted servers, how can you be sure it was written by the correct person?"

After they think for a while you can just say cryptographic signatures solve that. 

 “A bunch of servers” is already too much probably. “One server” seems to be the limit of understanding. But I guess I should try that simplified contrast more: Twitter has one server, nostr has multiple. 

 To non-techies of my age I tell: it’s like BitTorrent but for E-Mail. 

 Even if not technically correct (non-P2P), but they tend to know what a tracker is.