A Mod (short for Moderator) is a player level on the server. If you are looking for information on the server mods, then please see the server mods page.
A Mod helps other players and maintains a healthy atmosphere on the server, while ensuring that problems are resolved as quickly as possible. Normally Mods are able to whitelist players, although for now this ability is restricted to Admins.
Mods are chosen by Admin as they are needed, and as people demonstrate themselves capable of handling the responsibility. The quickest way to be rejected for the position of Moderator is to ask if you can become one, so just keep on playing and being an asset to the server!