The cdd, or central double decrease, takes three stitches and reduces them down to one. Instead of leaning to the left (like an sssk) or the right (like a k3tog), a cdd remains centered.
To work it, insert your right-hand needle into the first two stitches on the left-hand needle together as if to knit and slip onto the right-hand needle. Next, knit the next stitch on the left-hand needle as normal. Finally, lift the two slipped stitches (now on the right-hand needle) up and over the stitch you just knit, so that they drop off the needle.
The following video demonstrates how to work a cdd. The explanation and setup starts at 0:31, with the actual demonstration of the stitch at 1:10.