The Bifrost Bridge is a magic rainbow bridge in Asgard guarded by Heimdall the Gatekeeper. It is a link between all Nine Realms of the cosmos, but can be opened only if Heimdall permits it; the bridge is very powerful and it could destroy all realms if it is left open.
Thor Odinson, Loki Laufeyson, Sif and the Warriors Three use the Bifrost to reach Jotunheim and attack the Frost Giants by surprise. Later, Loki uses the Bifrost again to Jotunheim to ally himself with Laufey, and Sif and the Warriors Three use it to reach Earth and take Thor back to Asgard. In order to prove himself worthy of the throne of Asgard, Loki wants to use the Bifrost to
completely destroy Jotunheim and he opens it after freezing Heimdall and killing Laufey; during the battle of the brother Gods. Thor chooses to destroy the Bifrost, so preventing the annihilation of the Frost Giants. Loki falls from the bridge into a wormhole, unknown where. Thor believes that with the bridge gone, there is no way back to Earth and therefore to see Jane Foster again. However, Heimdall hints that there is still "hope".