Why Mastodon Is The Last Social Media You'll Need
First off the title may be a bit misleading. However people who have never heard of Mastodon have more than likely never heard of the fediverse. You have to show them the former in order to properly explain the latter. In my opinion anyway.
So Mastodon Then...
Imagine if email and Twitter had a baby. It would be Mastodon. Mastodon is similar to Twitter in that it is a micro-blogging platform where one can follow and be followed by friends or anyone else (depending on privacy settings). Since the platform is called Mastodon and uses a wooly mammoth/elephant as it's mascot your statuses are called toots. This is similar to the sound elephants make.
Mastodon is similar to email because of it's federation. What's that? Well, you may have a Gmail address and your friend may have an Outlook address. You can communicate with your friend using Outlook because Outlook, Gmail, and every other provider all speak the language of email. Mastodon servers also all speak the same language. It's called ActivityPub (more on that later). So just like email you can have an account on one instance (website) and your friend can have an account on another.