Yggdrasil, also known as the World Tree or the World Ash, is the meta-cosmological underpinning of the multiverse in Norse mythology. It is a gargantuan ash tree that floats in the middle of the cosmic void, and all of the Nine Worlds are either cradled amidst its branches or serve as the foundation whence one of its three enormous roots grows. One root reaches into Asgard, a second springs from Jotunheim, and a third lies in Niflheim, the realm of the dead. The Jotunheim root ends at the spring of all wisdom, Mimir's Well, which Odin sacrificed an eye to drink from, and also lies next to the cave of the Norns, the three goddesses of fate. The Niflheim root lies in a dark and noxious swamp infested with lindworms, with the greatest and most terrible being Nidhoggr, who seeks to kill the tree by gnawing away at its root. At Yggradsil's highest branch sits a giant eagle of unknown name, with a smaller (though still fucking huge) hawk named Vedrfolnir sitting on his head. The eagle feuds with Nidhoggr, and this constant battle is spurred on by the squirrel Ratatoskr. The branches are also home to four giant stags named Dáinn, Dvalinn, Duneyrr, and Duraþrór, who feed on the foliage.
In tabletop gaming, Yggdrasil is usually brought over as an analogue to the Astral Plane; it forms a kind of cosmic highway, and if you don't have a spell or a portal directly linking one world to another, you can get around the worlds by finding a portal to Yggdrasil and then exploring the giant tree-world until you find a portal leading to where you want to go.
Yggdrasil showed up in Planescape in just that sort of "cosmic highway" role.
In Scion, Yggdrasil is a legendary location associated with the Aesir pantheon.
In the Midgard setting, the cosmology is based around Yggdrasil.