The rainbow bridge connecting Midgard and Asgard is called Bifröst.
Bifröst is the divine bridge between Midgard, the human world, and Asgard, the realm of the Æsir gods. Norse sources describe it as a brilliant, burning bridge with three colors. The gods use it to travel to their daily assembly at the well Urðarbrunnr, near the roots of the world tree Yggdrasil.
The Prose Edda identifies Heimdallr as Bifröst’s guardian. Heimdallr lives at Himinbjörg, where the bridge reaches heaven, and watches for hostile jötnar approaching from the world beyond the gods. This association helps explain why Heimdallr’s horn, Gjallarhorn, is sounded at Ragnarök.
Bifröst is sometimes confused with Gjallarbrú, another mythic bridge that leads toward the realm of the dead. The bridge also has a different name in surviving sources: Ásbrú, meaning “Æsir’s bridge,” while the Poetic Edda records the form Bilröst. At Ragnarök, the forces of Muspell cross it, and the bridge breaks beneath them. Some scholars have interpreted Bifröst as a mythic image of the Milky Way, a rainbow, or possibly the aurora.