A BUP is the backup of an IFO file. It stores information similar to IFO file so that DVD players can use this file if the original IFO file can’t be read. BUP files tells the DVD player about the startup menu, location of video and audio tracks, and the start of chapters on DVD. BUP files can be opened with VLC Player.
BUP File Format - More Information
DVDs can get corrupted due to the possible scratching on its surfaces. This makes it difficult for a DVD drive to read them. If the IFO can’t be read due to scratches, the DVD players can’t play such a disc. In such cases, the media players resort to the BUP file that is an actual backup of the IFO file.