Runes are the Nordic symbols of writing used by the people of Cimmeria. They are related to the written alphabet of the Asgard, which is still used in Asgard documents and technology. Originally runes were ideographic, but they later came to represent letters and numbers.
On Cimmeria, the Asgard left the Riddle of the Runes in the Hall of Thor's Might as a test for the Cimmerians to prove sufficient wisdom to see the Asgard in their true form. The Riddle of the Runes represented:
The solution to the riddle was to use the numbers 3-14-15-9 to represent 3.14159, or pi, to be depicted as the radius on a geometric circle. By solving the Riddle of the Runes, Daniel, Carter and Gairwyn were the first to see Thor in his true form.
Cross Reference: Asgard, Cimmeria, Cimmerians, Gairwyn, Hall of Thor's Might, Kendra
Episode Reference: Thor's Hammer, Thor's Chariot