The World Tree. It's, according to the Eddas, a giant ash tree connecting a set of at least nine worlds. Midgard, which is where humans and dragons live; Asgard, where most of the gods and those who die in battle live; Vanaheim, where nearly all the rest of gods live; Alfheim, home of the elves; Svartalfheim, home of the dwarves; Jotunheim, the homeworld of the giants; Muspelheim and Niflheim, the worlds of fire and ice respectively; and Helheim, which is ruled over by the goddess Hel and is where everyone else goes when they die.

[Hiccup pauses for a few moments]

Also there's a giant eagle at the top of the tree and a giant dragon under it, and a squirrel runs up and down the tree passing insulting messages between them because it's a jerk.