Bifröst is the rainbow bridge that connects Asgard, the world of the Aesir tribe of gods, with Midgard, the world of humanity. Bifrost is guarded by the ever-vigilant god Heimdallr, who lives in Himinbjörg by Bifröst, and guards the bridge from mountain jotnar while sitting at the edge of heaven. During Ragnarok, the giants breach Heimdallr’s defenses and cross the bridge to storm Asgard and slay the gods.
The gods were said to have built the bridge from heaven and Earth. And due to its odd spectrum of colors, a mortal may mistake it for a rainbow. The bridge is said to consist of three colors, has great strength, "and is built with art and skill to a greater extent than other constructions."
Although the bridge is strong, it will break when the forces of Muspelheim attempt to cross it, and their horses will have to make do with swimming over "great rivers." While the bridge may be highly praised for its structure, it does appear to have a limit to how much it can keep out invading forces. Furthermore, the forces of Muspelheim are destined to destroy the bridge itself and thus it shows that even the bridge is bound by the prophecy of Ragnarök.
Every day the gods ride their horses across it, with the exception of Thor, to reach Urðarbrunnr, a holy well where the gods have their court. It is said that even the colors of the bridge have a purpose, such as the red in the bridge actually being burning fire, and, without it, the frost giants and mountain giants would "go up into heaven" if anyone who wanted could cross Bifröst.