The way it works is that there’s not one lemmy site, there’s many different sites, or “instances” that people can host on their own servers—lemmy.world and lemm.ee are two of these. The catch is that they are “federated”, meaning that a user in one instance can see, vote on, comment on, etc posts from another instance. That’s why you can be on lemmy.world and see posts from !memes@lemme.ml. So if lemmy.world goes down it’s as simply as typing in another instance to your address bar :^)