Each lemmy "server" or "instance" is the analogy of an email server. E.g. "lemmy.ml" or "beehaw.org" is equivalent to "gmail.com" or "hotmail.com" or "us.gov" or whatever email server.
You can use any email server to look at any email, the same way you can use any lemmy server to look at any lemmy post or community. You can use any email app to access any of your email accounts and view any email, the same way you can use any lemmy app to access any of your lemmy accounts to browse any lemmy post/commu ity.
If you currently use Gmail, and Google becomes evil, you can switch to Yahoo or Hotmail and still use email like usual. In the same way, if Lemmy.ml becomes evil you can switch to Beehaw.org and keep reading lemmy posts.
*** note that "lemmy" (the platform) and "lemmy.ml" (the instance) are not the same thing. It's kinda confusing, like if there's an "email" server called "email.com"