An easy analogy that common users can understand is e-mail. E-Mail is also decentralized, everyone has an e-mail address but everyone uses a different e-mail host (the domain name after the "@"). So e.g. "[email protected]" has an account at gmail.com but "[email protected]" has an account at mailbox.org. Both are completely different, yet they can communicate with each other. There's not one company controlling or storing every single e-mail account or inbox. It's spread out and everyone can choose the mail provider they like or trust the most.

Then you use that as a bridge to explain Lemmy, or Mastodon, or other Fediverse social media platforms. And remind the listener that single companies having full control over everyone's accounts is generally bad and opens the door for all sorts of abuse and manipulation or arbitrariness.