It’s like email. When you sign up for Gmail, you get all the Gmail features, use the Gmail website to access your email, and can send email to any other email, like Proton or Hotmail or whatever. But if Gmail goes down you can’t read or send any email.
With Lemmy, you can see communities from any other server, like lemmy.ml or tchncs.de. But some servers might have a different interface, slightly different features, and if your instance goes down you won’t be able to access log in unless you have an account somewhere else.
The extra cool thing is this extends beyond Lemmy. Some other social medias like Mastodon and Pixelfed communicate the same way that Lemmy does, they just look different. You can see Lemmy communities on Mastodon, and see Mastodon toots on Pixelfed.
There’s about as much difference between servers as there is between email providers. Sometimes there’s technical differences or some might be more reliable, but Lemmy will be Lemmy wherever you go.