A folder tree structure is just a series of folders, for instance on linux you have
and so on. Then you will have sup folders:
Now as your at school your probably on windows so you will have:
C:\Documents and Settings
Then as with linux you will have sub folders. This isn't just used for the root ( / or C:\) of the sytem but could be a folder structure of your home directory, work folder etc anything.
Hope this helps and im not rambling on.