Bifrost (pronounced roughly “BEEF-roast;” Old Norse 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 Heimdall. During Ragnarok, the giants breach Heimdall’s defenses and cross the bridge to storm Asgard and slay the gods.
The etymology of the word is uncertain. The original form of the name seems to be Bilröst, which suggests a meaning along the lines of “the fleetingly glimpsed rainbow.” If Bifröst is correct, however, the meaning would be something akin to “the shaking or trembling rainbow.” In either case, the word points to the ephemeral and fragile nature of the bridge.
All rainbows, of course, are “fleeting.” In the pre-Christian Germanic worldview, the invisible, religious modality of existence doesn’t lie in a realm of absolute remove from the material world, as in monotheistic religions. Rather, it lies within or behind the everyday, material world. (See Myth, Pantheism, and Animism.) The mythological image of Bifrost expresses the existential meaning that the rainbow carries in this perspective, and, accordingly, Bifrost lies behind and within any and every visible rainbow, each of which is a transitory and quaking bridge between the sky and the earth, between Asgard and Midgard.
Looking for more great information on Norse mythology and religion? While this site provides the ultimate online introduction to the topic, my book The Viking Spirit provides the ultimate introduction to Norse mythology and religion period. I’ve also written a popular list of The 10 Best Norse Mythology Books, which you’ll probably find helpful in your pursuit.
 The Poetic Edda. Grímnismál, stanza 44.
 The Poetic Edda. Fáfnismál, stanza 15.
 Simek, Rudolf. 1993. Dictionary of Northern Mythology. Translated by Angela Hall. p. 36.