The Twelve Tribes of Kobol references the historic origins of the humans of the Twelve Colonies. Thousands of years ago, the human race left their home of Kobol and took to the stars. They settled together in the star cluster of the Colonies, each claiming one planet as their own: Caprica, Aerilon, Tauron, etc. The current names of the planets comes from the ancient names of the tribes, the signs of the Zodiac: Capricorn, Aries, Taurus, etc.
Scripture and disputed histories also claim that there was a thirteenth tribe which also left Kobol. This tribe, however, did not follow the other twelve but instead settled far away on Earth. It is this tribe that Galactica attempts to find after their Exodus.