Really it’s just like subreddits, but with an additional level added on. So each instance has its own culture and rules, and then their own communities within them that may or may not overlap with communities on other instances.
So you might think you’re commenting on a post from lemmy.world/c/politics, which is pretty liberal and mainstream for an American forum, but it’s actually hexbear.net/c/politics, which is quite far left and has very active users, who respond to your comment completely differently.