FREDder (or sometimes FREDer) is a term in the jargon of the FreeSpace community. A FREDder is any fan who uses FRED, FRED2 or the advanced FSO version (FRED2 Open) to make home-made missions for the FreeSpace Universe.
The act of using the mission editor is called FREDding. A verb to FRED is also used by the FreeSpace community. To FRED means "to make a mission using FRED."
An alternate spelling of FREDding is FREDing. Both versions are widely used by the community.