I’m also very new to lemmy so forgive me if I’m a bit mistaken here, but I saw a decent analogy on the dbzero instance. An Instance is like a street and the communities are like businesses in that street. You can go to multiple streets (instances like lemmy.world, lemmy.ca, etc) and find a macdonalds (/c/technology) on each one. You can walk into each and expect to find the same-ish fries (content) but the people will be different, and if you talk to someone at macdonalds on 1st (technology@lemmy.world) you won’t hear a response from someon at the macdonalds on 2nd (technology@lemmy.ml).

EDIT: But of course importantly, those streets are adjacent to eachother (federated) so you (your account) can walk freely between them even though you live on 1st street (lemmy.world)

That’s how I’m trying to make sense of it anyway