This is a shortcut to the standard Windows Map Network Drive facility, which allows you to map a network drive so that it appears as a local drive. Such drives can be accessed directly by FolderSizes via the Folder Browser pane.
Important: In Windows Vista and later, the operating system associates mapped network drives with the security token of the user that creates them (and then limits access accordingly). In such cases, network drives mapped by one user may not be visible to other user accounts. Please keep this in mind when running FolderSizes as an administrator, as user account elevation (through Windows User Account Control) can create this scenario. If mapped network drives are not visible as you expect, simply re-map them from within FolderSizes to ensure their visibility in future sessions.