The Red Right Hand is a form of Glamour Failure where the right hand of a supernatural character is blood-stained or marked with some other variation of red coloring. The red right hand could be seen as the spiritual equivalent of bright patterns on venomous animals - a warning sign that something wrathful this way comes.
Exactly what it means is debatable, and the GM has some wiggle room. The term originated in John Milton's Paradise Lost, where the demon Belial refers to God's red right hand, which can strike down vengeance upon the devils should they push the war in Heaven too far. Belial was arguing for peace, or at least not provoking a second war in heaven. In this context the red right hand signifies divine wrath. By extension, the red right hand can be applied to all the Holy Trinity, the Angel of Death, or any agent of Divine Retribution. Jesus Christ sits on God's right hand, so such a symbol could be used to mark a Messiah. However, since the mark of the beast is placed upon the right hand (or forehead), the red right hand might mark the Antichrist or his followers. From there it's a short jump to the Four Horsemen Of The Apocalypse, Satan, and various demons and fallen angels.
As byzantine and contradictory as all that is, there's other uses of a Red Right Hand that make the whole topic even more complicated.
•The expression Caught Red-Handed.
•The Red Right Hand appears on flags of the Irish province of Ulster, and left red hand appear on the personal Coat of Arms of Ulster's Baronets. ◦The origin of this emblem may be from son of the celtic sun god Labraid Lámh Dhearg (Labraid of the Red Hand).
◦Another tradition has built up concerning a boat race to see who would become King of Ulster. O'Niell was losing, so he cut off his own right hand and hurled it across the finish line.
•Red Hand Day is an international commemorative day, every February 12, intended to draw attention to the plight of children forced to fight in war as soldiers.