A mod pretty much skims topics on his/her forum and makes sure everyone is following the rules. Pretty much goes: be reasonable. No blaitantly off topic posts, no obscene images, nothing overly argumentative, etc. Should this occur, the mod deals with this by either locking or deleting a topic. Should the behavior be recurring or excessive, this is reported to the mod board for further review of the member's connduct.
If you are interested, feel free to PM me with info of what you would like to moderate, especially if you would like to do one of the less active forums.