Slip 1, Knit 2 Together, Pass Slipped Stitch Over (sk2p)
The sk2p--or slip 1, knit 2 together, pass slipped stitch over--is a left-leaning double decrease. It takes three stitches and turns them into one stitch. You may also have seen this stitch abbreviated as sl1-k2tog-psso.
To work it, insert your right-hand needle into the first stitch on the left-hand needle as if to knit and slip it onto the right-hand needle. Next, knit the next two stitches on the left-hand needle together. Finally, lift the slipped stitch (now on the right-hand needle) up and over the k2tog you just created, so that it drops off the needle.
The following video demonstrates how to work a sk2p: