An admin is short for a Reddit site administrator. Reddit admins are employees of Reddit who could do any number of things including help keep the platform up and running, communicating changes, or enforcing Reddit’s rules and Content Policy.
Admins will have a red icon that looks like the Reddit mascot next to their username on their profile page and on their posts and comments.
If you have more questions that aren’t covered in these FAQs, check out r/help. Someone may have had the same question, or you can ask a new one and a Reddit administrator or one of your fellow redditors will help you out.