It returns that portion of the file path that corresponds to the portion of the URL that comes after the location of the .vuh file. That's pretty muddy, huh? An example?
Your package is mounted in the site map at /yourPackage. You have a file /packages/yourPackage/www/index.vuh, and you do not have an index.{tcl,adp,html} at that location since the RP would serve them up before finding index.vuh.
If your index.vuh were to log path_info on the following requests, it would appear:
http://host/yourPackage/ (path_info="")
http://host/yourPackage/admin/page?id=1 (path_info="/admin/page")