Hi there! We actually have a page containing all of our rules right here. It should help you figure out what kind of questions and answers are appropriate for this sub. Let me know if you need any clarification on that.

Aside from the rules, I would also plus this page which is guidelines for how to ask better questions!

To add a bit to the moderators' answers: in the Web interface, there is a sidebar on the right side of each page. Near the top, it has

New to /r/AskHistorians? Please read our subreddit rules and FAQ before posting!

where "subreddit rules" and "FAQ" are links (with one reddit Preferences setting, they are not distinguishable from text). In the Android app, this sidebar can be reached from the subreddit's main page, click the three dots at upper right, click on Community Info.