Pretty much it's like an e-mail service overall. obviously for the "MOST" part you can join any instance and it will let you view content on almost any other instance.

Like say beehaw I believe defederates from instances that don't have verification steps at the start.

instances like, lemmygrad, hexbear etc... are considered tankie, so a handful of places defederate them. and I believe a few like that tend to lean right and get defederated by most.

Edit: fixed misassumption on what hexbear was,

Don't worry too much about the instances, for the most part they are not that important. There are a few outliers tho:

• lemmy.ml, lemmygrad.ml and hexbear.net are politically in the Authoriarian Left extreme. Unless you either are politically close to them or like getting dunked on by people who think China and Russia are Heaven on Earth, don't go there. Sadly the main devs for lemmy are situated on lemmy.ml.
• beehaw has a curated experience by blocking any instances which do not verify registrations (including the largest one at lemmy.world) and a different discussion culture than most. It's a very chill place tho.
• and then you can also join instances that use PyLova (a fork of PieFed caused by ... not cool actions by PieFeds dev, especially ironic since PieFed already exists partially because of lemmys roots on lemmy.ml), e.g. quokk.au. These give you a different interface and features, while giving you the same access to content.

There are also a bunch of outlaw instances, either far-right or tolerating CSAM. These you can only stumble upon if you actively search for them - they are defederated in a pretty coordinated effort, simply because due to the way lemmy works, their content would be replicated, creating a legal nightmare, and of course no respectable admin wants to touch that stuff.

Id say what are your preferences.... If you want mainstream for main stuff, Lemmy.world. if you want piracy or anarchy, lemmy.dbzer0.com is great. If you are from Canada Lemmy.ca is good. What's bad? Opinionated. .ml is large but has backlash, .hexbear sees a lot of backlash, but not knowing you... It's open game

the tankies, like another user mentioned, are very hyper-political authotarian with a "left wing narrative", if you imagine conservatives, this would be the exact opposite(with the exact same authotarian nature).