It’s like if Facebook, Reddit, and Twitter all had the capacity to import content from each other. Then add in that most of the Fediverse’s "facebook, twitter, and reddit"s aren’t run by centralized institutions. You can sign up for an account with any of them, but the administration of the one your account is on gets to decide which other sites content it wants to import. If you don’t like the way the admins are deciding on content in one, you can again start your own or just hop to another one. The Lemmy instances are forum like in nature similar to reddit, Mastodon instances are like twitter.