A DDP (Disc Description Protocol) image is a kind of disk image of your CD.
 
The DDP image will encode all the information that was made in the editing of your album during your mastering. This obviously includes the sound as well as the fades in, fades out and the transitions between each title but also the metadata: cd text, ISRC codes and UPC/EAN code.
The pressing plant will use the DDP image to produce the Glass Master which will be used to press the CDs.