Yggdrasil is the name of the tree that links the 9 different worlds of Norse mythology. The tree is an ash. It links Niflheim, Asgard, Midgard, Vanaheim, Alfheim, Nidavellir, Svartalfheim, Jotunheim, and Muspelheim. Below the tree resides Hel as well as the Well of Urd.
As an example when Sjöfn wields Yggdrasil she then is not just a minor goddess of love. She is now able to heal those who are injured or dying. She can reach a power so far inside of herself that she would have never known about when holding Yggdrasil.