You know who they are. They're that one guy that the Game Master keeps--that one you never wanna cross. They may be the final boss shown before you can fight it or the GM's hand In Character, but you know that the second you step forward, you get the smackdown.
It's sometimes called the similar term "godmodding". Saying that somebody "God Modes" in a game is to suggest that he/she basically refuses to allow anything that negatively impacts him. However, the term "godmoding" is viewed a bit more neutrally than "godmodding" and could just as easily refer to when the characters are given a free pass on potential death for whatever reason. While death is not that common amongst PCs, this is something that utterly destroys suspension of disbelief. This is the kind of person that will cause the players to rise up against the GM--until said GM uses his one-man-army to destroy them.