The working tree (or working area) is a root directory and all its sub-directories where the work on your files happens (extraction, modification, commit).
The location of the root work tree is:
It's called a tree because the directory is synchronized by git in the git repository as a tree object.
It's an file system tree (area) where:
If you want to your working directory back to the status of upstream (server repo), the following commands can be used (remember everything you've worked on is gone after these):
git reset --hard upstream/master
git clean -fdx