Off the top of my head...

1. Removing spam
2. Reviewing flagged posts and actioning them as needed
3. Ensuring that posts follow the community rules
4. Organizing events and contests for your userbase, if applicable

It can definitely be a thankless job at times, but also very rewarding to see the community flourish.

I imagine the types of actions you'll do is similar to what can be seen in sh.itjust.works/modlog . It'll be less work in the beginning when you have only a few users, but no one's gonna blame you if you stop. Well maybe you'd want to pass the torch to someone first.

I realize this isn't an ELI5 explanation, but https://join-lemmy.org/docs/en/users/04-moderation.html is a good, basic primer on moderation in lemmy.

Not specific to Lemmy, but basically you remove spam, offensive posts/comments and cultivate the space the way you think it should be.