The Triple Horn of Odin is made of three interlocking drinking horns, and represents Odin, the father of Norse gods. The horns are significant in the Norse eddas, and feature prominently in elaborate toasting rituals. In some stories, the horns represent the three draughts of the Odhroerir, a magical mead.
According to the Gylfaginning, there was a god named Kvasir who was created from the saliva of all the other gods, which gave him great power indeed. He was murdered by a pair of dwarvse, who then mixed his blood with honey to create a magical brew, the Odhroerir. Anyone who drank this potion would impart Kvasir's wisdom, and other magical skills, particularly in poetry. The brew, or mead, was kept in a magical cave in a far-away mountain, guarded by a giant named Suttung, who wanted to keep it all for himself. Odin, however, learned of the mead, and immediately decided he had to have it. He disguised himself as a farmhand called Bolverk, and went to work plowing fields for Suttung's brother in exchange for a drink of the mead.
For three nights, Odin managed to take a drink of the magical brew Odhroerir, and the three horns in the symbol represent these three drinks. In the prose eddas of Snorri Sturlson, it is indicated that at some point, one of the dwarf brothers offered the mead to men, rather than to the gods. In many parts of the Germanic world, the triple horns are found in stone carvings.
For today's Norse pagans, the triple horn often is used to represent the Asatru belief system. While the horns themselves are certainly phallic in symbolism, in some traditions the horns are interpreted as containers or cups, associating them with the feminine aspects of the Divine.
Odin himself is portrayed in a number of pop culture sources, and his drinking horn often makes an appearance. In the movie The Avengers, Odin is portrayed by Sir Anthony Hopkins, and drinks from his horn in a ceremony honoring his son, Thor. Odin also appears in Neil Gaiman's novel American Gods.