Forewarning to all ye who enter here: Thar be spoilers on this page.  So in the event you have somehow not yet read or watched anything of the Potterverse, you have been warned.  Let us proceed.  

The World Tree, in its many forms, is often depicted with the Overworld, the branches, representing the future, the spirit realm, and sometimes thought and possibility.  These are all unknown quantities most of the time and often thought to be subject to change based on current events.