In the Gensou Suikoden series, they are 27 runes that were created as a result of the legendary battle between the Sword and the Shield, which also created the world. They wield immeasurable power and are sentient, choosing their masters at will. When attached to a host, they will appear in the form of a tattoo-like symbol, typically on the host's hand or forehead.
All of the lesser runes that grant magical powers or other special abilities in the games are "descendants" of the True Runes. Some of the more powerful ones are also sentient, but to a lesser degree than the True Runes themselves.