In Norse mythology, Bifrost is the bridge between Midgard, the realm of man, and Asgard, the realm of the gods. Since it is the only way for the giants to enter Asgard it is closely guarded by Heimdall, the watchman of the gods.
Bifrost was made of three colors with magic and great skill by the Aesir and is incredibly strong. It is also called Asbru, referring to its makers. At the end of the cosmos, this rainbow-bridge will collapse.