A BUP file is a backup of an IFO file found on a DVD that has no use on its own. It is a file required on all DVD video discs, and it is a copy of the IFO file required to play the DVD video.
When a DVD disc is copied to hard drive, the main video file is accompanied by two other file types having the IFO extension and BUP extension. IFO files inform the DVD player about the disc's chapters as well as how to navigate the video. A BUP file is an exact copy of the IFO file, and it is used by the DVD player if the IFO file is damaged or cannot be read.
Copy the BUP file and rename it to the IFO file that is not working. Windows Media Player can open the IFO files to play the VOB files containing the video stream details.
Some versions of Windows do not support DVD playback by default, namely the Home Basic version. For these versions, additional software that supports DVD playback is needed. Alternatively, try using a different media player that supports VOB and IFO files without the need for a third party codec, such as VLC Player.