What's the difference between a hero and a mod?
The difference is below. However, the more important point is that anyone can work to help make the overall experience of Socratic, for both Contributors and Questioners, better.
I believe the current difference is as follows:
a Hero is someone who is designated to reach out to the Contributor community, initiate conversations, help welcome new Contributors - in short be a resource for the Contributor community.
a Featured Answer Reviewer is someone who looks at the content being produced by Contributors and decides what should be marked as a Featured Answer.
a Moderator is someone who handles safety (protects the community from harmful content and abusive users) and basic site maintenance (deleting answers, moving or deleting questions, and so on).
In short, a Hero interacts with Contributors, a Featured Answer Reviewer interacts with content, and a Moderator deals with under-the-hood stuff. So that is the short on what those flags mean.
The more important point to make is that these roles are not the exclusive providence of those with a certain title or flag. In fact, I'd argue that one of the great strengths of Socratic is that everyone can contribute - whether by editing answers, welcoming new Contributors, sharing new ways to do things - these are all great ways to make the overall experience of Socratic, for both Contributors and Questioners, better.