Reddit glossary
What is Moderator (mod) on Reddit?
A moderator is a volunteer who runs a subreddit, sets its rules and removes posts that break them.
Mods are normal Reddit users, not Reddit employees. They look after their community in their spare time. They can remove posts, remove comments, ban people and change the rules.
You can message them through modmail, a private inbox that belongs to the subreddit. It is the right place to ask if a post is allowed before you write it.
If you are promoting something
Treat mods like the owners of a venue. Ask first when you are not sure. A polite message before posting can save you a ban later.