Trees often correspond to life, knowledge and the element earth. Yggdrasil is a more specific, non-garden variety tree. It directly correlates to connectivity to spiritual realms, wisdom and spiritual journies. If you have Norse or Scandanavian blood in you, the tree might relate to your "roots" so to speak. Even if you are not related to anything Norse, perhaps the qualities and characteristics you associate with Vikings and such relates to aspects of your personality.
I don't know how you do with meditation, but if you are confident enough to relax for 15-20 minutes in a quiet space try this: Spend about 5 minutes just relaxing. Get yourself to the point where you feel totally calm and at peace, perhaps to a point where you feel like you might even fall asleep. If you do, don't worry, you might dream of soemthing relavent and if you don't you can always try again. The idea though is to get yourself into a semi-altered state of mind where you can conjure images and explore details without much effort. If you can get this far, try to explore the tree in detail. Think of this as looking at the tree under a microscope, or looking at a planet through a telescope vs the naked eye. By doing this you may uncover things you never knew of before.
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