Explain that the fediverse is the 2nd thing, and that because it is open source and community maintained you don't need the 1st thing i.e. the company which makes the 3rd thing, the actual community and social network, able to be semi-decentralized and not controlled by one corporation and/or government.

The way this is accomplished is there are lots of individual communities/social networks that all "federate" together based upon a common protocol (called ActivityPub) so that interaction can happen between users from different communities without it being an issue.

Explain that lemmy and piefed are "reddit-likes", mastodon is a "twitter-like" and pixelfed is a "instagram-like" as examples of what the fediverse version of common corporate social networks look like.

Most importantly, emphasize over and over again that nobody cares if you walk away from your fediverse account and don't engage for months, nobody is trying to shove ads down your throat or pay off a massive Venture Capital investment, while people do often contribute to support their community after they decide they really like it, there is no monetization driving the whole network, people show up because they want to be here, that is it. Further if you don't like the way a community is run there are real actual alternatives, it isn't a binary thing like corporate social media.