Rainbow Bridge is literally that; a bridge in the appearance of a rainbow.
Odin was seeing riding up the Rainbow Bridge upon Sleipnir, after recovering his eye from Goliath in Norway. ("Eye of the Storm")
Real World Background
The Rainbow Bridge is clearly Bifrost, the bridge from Midgard (the human world) to Asgard (the home of the Aesir) in Norse mythology, which took the form of a rainbow. (The red hues of the rainbow are supposedly fires that run along the bridge to discourage the frost giants from invading Asgard.) At Ragnarok, when Surtur the fire demon and his followers, the Sons of Muspell, ride across Bifrost to assail Asgard, the bridge will break beneath them. (If this has already happened in the Gargoyles Universe, the bridge must have been repaired.)
- Bifröst at Wikipedia, the Free Encyclopedia