I just told my fairly tech-unsavvy partner the email analogy:
You sign up on Google, I sign up on yahoo, my bro-in-law runs his own from a server in his house. We can all email each other and the email looks mostly the same no matter who reads it, but yahoo isn't Google isn't my bro-in-law. Lemmy = email in general, yahoo = lemmy.ml, Google = lemmy.world, etc.
She immediately got it and has an account on some instance and has subscribed to a bunch of places.