Bifrost - The Rainbow Bridge
The burning rainbow bridge Bifrost connects Midgard to Asgard, guarded by Heimdall.
Bifrost is the bridge connecting the realm of humans to the realm of gods. It appears as a burning rainbow and is guarded by the ever-watchful Heimdall. Bifrost represents the connection between mortal and divine, the path of spiritual ascent.
The bridge burns with fire, for giants would storm Asgard if the way were easy.
Bifrost symbolizes the guarded path between human and divine realms.
Prose Edda