The Windows File Manager was the file manager included with Microsoft Windows starting with version 3.0 until Windows 95 and NT where it was phased out in favour of Windows Explorer. The Windows File Manager was used to perform file management and hence replaced the functionality of MS-DOS (earlier versions of Windows had a simple file manager called MS-DOS Executive).
Why are we talking about File Manager? Well an updated version has been released that will work on Windows 10! You can download it here if that kind of thing floats your boat.