Yggdrasil was the Norse world tree connecting the nine realms.
In Norse mythology, Yggdrasil is usually described as a vast ash tree whose branches and roots extend through the cosmos. The tree links the worlds inhabited by gods, humans, giants and other beings. Important locations associated with it include Asgard, Midgard and the underworld region governed by Hel, although the exact organization of the nine realms is not presented as a single complete map in the medieval sources.
The Poetic Edda and Prose Edda describe several beings connected with Yggdrasil. An eagle is said to live near its crown, the squirrel Ratatoskr carries messages along its trunk, and the dragon Níðhöggr gnaws at its roots. The Norns, often identified as Urd, Verdandi and Skuld, shape fate near the tree.
Yggdrasil is often confused with Bifröst, but they are different. Bifröst is the rainbow bridge connecting the human world and the realm of the gods. Valhalla is Odin’s hall, not a cosmic tree or bridge.