This field determines where the URL path above will be redirected to. You have two different choices :

URL at other website
  When this option is selected, any browser accessing the URL path will be redirected to this new URL. If a sub-directory of the path is accessed, the redirect will also include that same path.

Directory on this system
  When this option is chosen, any browser accessing the URL path will be served content from the specified directory. This should be in the domain's home directory, and preferably under `public_html` or else PHP scripts are unlikely to work.