You're already on fedi, so I assume it wont tell you more than you already know, but the way I explain it using that method generally goes something like
Let's say you're on Facebook and a friend is on Twitter. You want to send them a message, but you can't send messages from FB to twitter, right? That'd just be weird right? (Generally people agree that this is indeed the case and would be weird otherwise)
Of course you can also call them on the phone. Let's say you're with a telephone provider BlablaFon. A friend also has a phone, but they are with another provider, namely Phonica (I'm using fake names here, typically I use existing names they know because that parses easier).
Now image that you're calling them, but it doesn't work and you get told that it's normal it doesn't work because they are with another provider. That be weird right? You both have a phone and a subscription, you should be able to call each other, right? Why would a provider not allow calls to another provider? (Generally they agree that this would indeed be weird)
Yet, when we want to contact each other from one social media account to another, we suddenly not only think it's normal that it doesn't work, we'd even think it's strange if it would work! But it works for phones, why would it not work for social media either? (Here I typically see in their facial expressions that something clicks for them)
So that's basically what the fediverse is. Just like you have several telephone providers, we have different social media platforms. But even if you're on a different platform, you can still talk to each other like you're part of one big network. Or better put, you *are* all part of one big network, namely the fediverse.