http://www.google.com/search?hl=en&q=robots.txt+tutorial
The "root" is always the top level of anything.
The "root" of your hard drive is the area where you can put files that is
not inside any folders.
The "root" of the web site is the area where you are not inside a folder;
so, "yourweb.com" rather than "yourweb.com/somefolder/"
(The terminology comes from the "tree" diagram of folders, each folder is a
branch, the files are the leaves and the "root" is the part where there are
no branches yet. They could have said "trunk" ... but that implies a
container in other useages so it is not a good a term as "root".)
