“Bifröst (Old Norse bifrǫst, literally the “tremulous way”, from bifask “to tremble” and rǫst “a distance”) in Norse mythology is the bridge leading from Midgard, the realm of mortals, to Asgard, the realm of the gods, which the gods travel daily to hold their councils and pass judgments at Urdarbrunn (Well of Urd) under the shade of the tree Yggdrasill. The bridge itself is the rainbow and its guardian is the god Heimdallr, whose hall of Himinbjorg is located at the upper end of the bridge. The red color was the flaming fire, which served as a defense against the giants. The bridge is destroyed at the end of the world, Ragnarök. It was built by the Æsir.
Much of what we know concerning Bifröst comes from Snorri (quoted below), but mention of it is also made in the Poetic Edda. For example, in Grímnismál, stanza 29 mentions Thor crossing over rivers (such as the Körmt and Örmt and the “Kerlaugs twain”) which boil from the fire of Bifröst each day on his way to the judgement place at Yggdrasil, and later in stanza 44 it is named as the best of bridges in a list of the foremost of things (including Yggdrasil of trees, Sleipnir of horses, etc). In Fáfnismál, stanza 15 (as well as Snorri) indicates that it will collapse when ridden upon by the fire giants during the destruction of Ragnarök.
Some scholars, including Ake Ohlmarks and Franz Rolf Schröder thought that Bifröst may have originally represented the Milky Way and was reinterpreted by Snorri as a rainbow when confronted with variations in terminology.
His alternative names include Bilröst, Ásbrú, Bifrost (anglicized form).”