The site you make your account on is your home server (instance). That server takes on the job of processing your data when you use lemmy. Your home server is lemmy.world, mine right now is lemm.ee
If a lot of people use the same home server, it gets expensive for the team running that server, the meme is about lemmy.world having issues with downtime because of this. You can have whatever home server you want and still access anything on the fediverse, so if you join a smaller instance as your home server, by literally making a new account on a different lemmy site, you can still use lemmy exactly the same (use your app to look at all instead of local) but spread the load so big instances like lemmy.world don't have too much strain. Doing this is good for the long term health of the fediverse.
I've been pretty happy on lemm.ee. I had an account on .world but set this up when I had issues connecting, my original was .ml which I switched from for similar reasons. Here's hoping lemm.ee can remain stable going forward, I'm trying to use the same username for consistency but its a pain losing comment history/ account age when hopping instances.