I guess it depends how tech illiterate they are and where their confusion stems from, but as long as they know what email is, then I like to explain anything in the fediverse by using email as an analogy:
You know how, with email, you don’t have to care about whether the person you email is using Gmail or Apple or Yahoo or whatever? Email is the kind of message you send, and whoever gets it can use any email app to look at it and reply to you.
(Insert federated platform name here) works the same as email. You want to post or reply on (Insert federated platform name here)? Use whatever client you like, and it doesn’t matter if the people you’re communicating with use the same client.
It’s not strictly a perfect analogy, but it’s close enough to get over the usual confusion where people have been conditioned by corporate social networks to expect services to have one and only one blessed app.
There are a few:
I’m not a user of any of those, but from descriptions and screenshots alone they look decent.