Horgr is a type of altar, constructed of piled stones, used by the Vikings in religious and civil ceremonies.
A typical use of the horgr would be to place upon it a bowl of the blood of an animal sacrificed to a Norse deity (e.g. a goat for Thor, a sow for Freya, a boar for Freyr), then dipping a bundle of fir twigs into it and waving the bundle in the form of the "hammer-sign" to spatter the participants with the blood. This would consecrate the attendees to the ceremony, such as a wedding.
Like Judeo-Christian and other traditions, the Norse religion vested great spiritual significance in blood; one legend speaks of a horgr built to Freya by Ottar, and when it was "reddened" with blood, she burned the stones to glass.