Short answers, pulled from the story.
Bifröst is a burning bridge in Norse mythology that connects Midgard, the realm of humans, to Asgard, the realm of the gods. It is described as having three colors, including a red band of burning fire, and is guarded by the god Heimdallr at its heavenly end in Himinbjörg.
Scholar Andy Orchard suggests Bifröst means "shimmering path," connecting it to the Old Norse verb bifa meaning "to shimmer" or "to shake." Austrian Germanist Rudolf Simek offers "the swaying road to heaven" as another possible meaning, and if the alternate name Bilröst is the older form, it may mean "the fleetingly glimpsed rainbow."
Bifröst breaks apart when the sons of Muspell ride across it during Ragnarök. The Prose Edda quotes High saying that nothing in the world will be secure when Muspell's sons attack, and the Poetic Edda poem Fáfnismál describes the bridge breaking as the gods cross it, leaving their horses to dredge through an immense river.
The god Heimdallr guards Bifröst. He lives in Himinbjörg, described as standing at the edge of heaven where Bifröst meets it, and sits there watching to prevent mountain jotnar from crossing the bridge.
Scholars have proposed Bifröst represents the Milky Way, a rainbow, or the Aurora Borealis. The Aurora Borealis fits the descriptions of fiery quality and three colors more closely than a rainbow, which is stable, not fiery, and does not end in the heavens.
Bifröst appears in Richard Wagner's 1869 opera Das Rheingold, in J. R. R. Tolkien's legendarium in the form of the "level bridge" of "The Fall of Númenor," in Marvel Comics and the Marvel Cinematic Universe film Thor where it is described as an Einstein-Rosen bridge, and in the game Hotline Miami 2: Wrong Number. The Rainbow Bridge is also widely invoked in contemporary culture to reference a deceased pet.