- Look at the mod queue, replying to any mod mail we've received and any posts that have been automatically removed by our AutoModerator script.
- Have a peak at the new posts, on any one day my sub-reddits only get a couple of hundred, so it takes a couple of minutes for a cursory glance.
- List all reported messages and see whether they're acceptable.
Then, because most of the sub-reddits I'm currently contributing to are about projects I'm working on, I generally answer any questions people have or problems they're facing.
However, this is just for me. It really truly depends on the type of sub-reddit you're looking to moderate as to what work you're required to do.
More generally, I think the moderators' main job is to set the mission/rules of the subreddit, and enforce them. Subreddits with weak enforcement rarely thrive as they devolve into the least common denominator.