An S2KP, or slip 2, knit 1, pass the slipped stitches over (which may also be abbreviated sl 2 k1, p2sso) is a basic double decrease. It's called a centered double decrease because it doesn't really lean to the left or the right; instead, the slipped stitches recede and the center stitch becomes prominent.
To being an S2KP, work to where the decrease needs to be and slip two stitches as if to knit, slipping them at the same time. This is the same sort of move you make to begin a knit two together.
