[justpctech]

the root directory is the very bottom level of anything. so for instance on your phone it would be \ or whatever your device is called or whatever the file explorer thinks it is. on my mogul, using the stock Sprint rom file explorer, the root would say My Device at the top. depending on what kind of file you are talking about, it "could" mean the root of Windows which would be the main Windows folder. other file explorer programs could show root as just the back-slash \ , so the root of Windows would look like \Windows.
so it sort of depends on what file explorer you are using and what root you are looking for.
hope that helps some.