A mod is a moderator.
Mod is actually short for moderator, a person who is responsible for keeping a watchful eye on the site.
In a nutshell, a moderator makes sure that the site is safe and that things stay peaceful in the community. Moderator work is pretty much maintenance work, under-the-hood stuff, so I won't go into too much detail about that here.
The bulk of my activity as a mod is not visible to the community, but I'm always around to help people with various problems or questions that they might have regarding Socratic.