Instances are servers running Lemmy software that communicate with each other using the ActivityPub protocol (on top of other Internet protocols). Members of one server (instance) can read and comment on posts on another Lemmy server (instance) without leaving their server. I hope that helps.

I’m guessing also you’re using the abandoned android client that isn’t open source. Here are the current apps for lemmy.

You can check out join.lemmy.ml for a list of instances and join.lemmy.ml/apps for a list of apps that work. Like @dessalines@lemmy.ml said if you’re using the lemmy app from the play store it’s broken and has been abandoned, so it’s not going to work unfortunately.

Lemmur is an app that’s really good but the devs aren’t ready to put it on the play store yet. If you have the f-droid app store you can install it from there soon. I’d reccomend using the mobile website (which is really good, the devs have done a fantastic job on it)

So basically you choose your preferred instance and also you can read other instances to some extent from your chosen one. Pretty much like email. Though I’m looking forward to even more decentralised future like what is being discussed in the Matrixverse.

if you are registered on lemmy.ml then you should be able to just enter your username and password.
lemmur is still being developed so it might not work perfectly.
you can also use a regular browser to access the site.

Update: Thanks for all the info, it makes a bit more sense now, and I’ve logged in after installing the Lemmur app

I was curious to understand how federation works with lemmy and this page ended up providing me with the information I was looking for.

I’m still not sure how to open a post from another instance in my current instance though

we also invite you to an instance of Lemmy which is szmer.info