Short answers, pulled from the story.
Yggdrasil is an immense ash tree that serves as the structural anchor of the entire Norse universe and supports all of reality. This single tree functions as the central axis around which the Nine Worlds revolve, connecting the heavens, the earth, and the underworld into a single interdependent system. The tree is described in the Poetic Edda and the Prose Edda as the holiest place where the gods gather daily to hold their governing assemblies.
The generally accepted meaning of the Old Norse term Yggdrasil is Odin's horse, a kenning that refers to the gallows upon which the god Odin once sacrificed himself. In the poem Hávamál, Odin describes hanging from a tree for nine nights, pierced by a spear, without food or drink, to gain the knowledge of the runes. Some scholars argue that the name originally referred to Odin's literal horse, Sleipnir, before the term was reinterpreted to describe the world tree.
The first root of Yggdrasil extends into the heavens, reaching the well Urðarbrunnr where the Norns dwell. The second root reaches down into the realm of the frost jötnar where the well Mímisbrunnr lies, and the third root extends into the underworld, the realm of Niflheim, where the spring Hvergelmir bubbles beneath the root. These three roots create a vertical axis that connects the highest heavens to the deepest underworld.
A squirrel named Ratatoskr scurries up and down the tree carrying malicious messages between the eagle perched at the top and the dragon Níðhöggr at the bottom. The eagle possesses great knowledge and the hawk Veðrfölnir sits between the eagle's eyes, while four stags named Dáinn, Dvalinn, Duneyrr, and Duraþrór consume the highest boughs of the tree. The dragon Níðhöggr and countless other serpents gnaw at the roots, trying to bring the tree down.
The end of the world known as Ragnarök is foretold to begin with the shaking of Yggdrasil, signaling the collapse of the cosmic order. In the poem Völuspá, the völva describes how the tree will tremble and nothing will be unafraid in heaven or on earth as the forces of chaos rise to destroy the world. The tree's destruction is a necessary part of the cosmic cycle, allowing for the rebirth of the world and the emergence of new life.
The concept of Yggdrasil as a world tree may have origins in the shamanic traditions of northern Eurasia where a central tree serves as a ladder to ascend the heavens. Hilda Ellis Davidson notes that the conception of a tree rising through a number of worlds is found in northern Eurasia and forms part of the shamanic lore shared by many peoples of this region. The image of the central tree in Scandinavia may have been influenced by the Pole Star and the tree may have been a natural symbol for the Germanic peoples.