CDD – Central Double Decrease is exactly what is says on the box. A double decrease which is central, i.e. it neither leans to the left or to the right but the stitches are decreased evenly, one each side of a central stitch.
Or, in other words, this decrease reduces three stitches down to one, in a vertical line.

As with all the double decreases, they are occasionally used in shaping but more often in stitch patterns.

Here is a quick photo tutorial on how to work this interesting decrease.