The Master File Table (MFT) is the list maintained by Windows on any NTFS volume that describes the location of files at the block level.
Any question about working with the MFT in an administrative context, not a programming context should be tagged with this. More general questions should probably be tagged [NTFS] or just [file-system]. The most common problem users and administrators are likely to have to deal with is a corrupt MFT, however a very fragmented one might also cause trouble.
Further reading at Microsoft's MSDN: http://msdn.microsoft.com/en-us/library/aa365230%28v=VS.85%29.aspx and http://msdn.microsoft.com/en-us/library/bb470206%28v=VS.85%29.aspx
