A DDP Player will play a DDP file. DDP is typically used as the master for CD replication, but DDP can also be a useful tool for approving projects because the songs will stay perfectly in order, with the exact song spacing and transitions, and it allows you or your clients to see the CD-Text info such as artist name, album title, track titles, ISRC codes, etc. This is not always true if you simply send WAV files to be auditioned in iTunes or Windows Media Player for example. Things can and will go wrong.
I would use The Google to learn more about what a DDP file, DDP image, or DDP file set is (all the same thing, just different wording) and then the DDP Player may make more sense to you. Pretty much any mastering DAW can make DDP files, but it’s not something you see in a multitrack recording/mixing DAW such as Cubase or Pro Tools.