Source Server description by Microsoft
Source server enables a client to retrieve the exact version of the source files that were used to build an application. Because the source code for a module can change between versions and over a course of years, it is important to look at the source code as it existed when the version of the module in question was built.
Source server retrieves the appropriate files from source control. To use source server, the application must have been source indexed.
Source: http://msdn.microsoft.com/en-us/library/windows/desktop/ms680641.aspx