The centered double decrease (cdd) does just what the name says — you start with three stitches and decrease down to one. The resulting stitch is centered, which means it doesn't lean to the right or the left. You may also see this stitch referred to as s2kp, or slip 2, knit, pass — this is just another way to describe the same stitch!
1. Slip the next two stitches knitwise together onto your right needle.
2. Knit one stitch, then slide your left needle through the fronts of the two slipped stitches. Pull the slipped stitches over the knit stitch, and drop them off the right needle.