The rainbow bridge that connects Asgard to Midgard is called Bifröst.
In Norse mythology, Bifröst links the gods’ realm, Asgard, with Midgard, the human world. The Prose Edda describes it as a brilliantly constructed bridge with three colors and a fiery section. The gods cross it to reach their assemblies near the well of Urðarbrunnr, while Thor usually wades through rivers instead.
Heimdallr guards Bifröst from his home at Himinbjörg, where the bridge reaches the heavens. During Ragnarök, the bridge is fated to break when the forces of Muspell cross it. This makes Bifröst more than a peaceful route: it is also a vulnerable boundary between cosmic realms.
A common mix-up is Gjallarbrú, another mythic bridge. Gjallarbrú crosses the river Gjöll on the route to the realm of the dead, whereas Bifröst is the gods’ bridge between Asgard and Midgard. The older poetic form of the name is Bilröst; Bifröst is the better-known Prose Edda form.