Basically everything goes through your instance. If you make a post, it goes to the copy of the community that's on your instance. Likewise if you comment. If you join a community, your instance starts listening for changes and stores those on the instance.

That way if another instance goes down, you still have a copy of all of the content there that someone on your instance is interested in. So that way pretty much everything is backed up.

I personally think we can do better, but it's an easy enough system that all but guarantees that content doesn't disappear. You could even set up an instance that never deletes anything if you want to make sure you don't lose any data.