slip 2 stitches together knitwise, knit 1, pass 2 slipped stitches over
A CDD simply stands for centre double decrease. This can also be called a s2kp2 which reads as slip 2, k1, pass 2 slipped stitches over.
To work this centre double decrease, start by inserting your right needle into the first 2 stitches on your left needle, slipping them off onto your right needle together, as if to knit.
Next simply knit the next stitch on your left needle. Using your left needle pick up your 2 slipped stitches together and passing them over the top of your knit stitch and off your needle.