Instances are like Reddit, Twitter, instagram, etc. Each instance has certain rules that may vary from the parent they forked from (see beehaw vs Lemmy.ml). Those instances will then have a structure like subreddits, posts, comments, etc.

Lemmy is similar to Reddit. Communities (c/*) are subreddits

Mastodon is similar to twitter. Microblogs are tweets.

Kbin combines Lemmy and Mastodon.