Yggdrasil is the name of the tree that links the 9 different worlds of Norse mythology. The tree is an ash. It links Niflheim and Asgard, amongst others. Below the tree resides Hel as well as the Well of Urd.
Yggdrasil is the tree of life between all 9 world's of Norse Mythology. life flows through Yggdrasil so those who can create and grow life (goddesses) Yggdrasil makes them stronger giving goddesses powers held deeper within themselves.
As an example when Sjöfn wields Yggdrasil she then isn't 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