what is vss for visual studio 2008

VSS is used for stroing versioning files of your project. Once installed VSS in you can set your project in VSS for day by day versioning

Those initials probably refer to Visual SouceSafe which is a version control system written by Microsoft (see http://msdn.microsoft.com/en-us/library/3h0544kx(VS.80).aspx for more details).