ResetDirectory[ ] resets the current working directory to its previous value.
Successive calls to ResetDirectory yield earlier and earlier current directories.
ResetDirectory uses the directory stack given by DirectoryStack[ ].
ResetDirectory removes the last element from the directory stack, and makes the second-to-last element current.
See The Mathematica Book: Section 2.11.4 and Section 2.11.4.
See also: SetDirectory, Directory, $Path.