S2KP stands for slip 2 sts, knit one, then pass slipped stitches over.
A S2KP is a centered double decrease, where you slip two stitches, knit one stitch, and then pass the slipped stitches over the knit stitch. This makes it similar to the SK2P, in which you slip one stitch, k2tog, and then pass the slipped stitch over the k2tog. By “centered double decrease” I mean that this is a decrease that turns three stitches into one (decreasing two), and also does not lean to the left or right.
Slip two stitches separately as to knit, k1, pass slipped stitch over then going into slipped stitch from left to right, use the left needle tip to pass the slipped stitch over the k1 and off the right needle.
Slip two stitches as to purl, k1, pass slip stitch over by using the left needle tip, going into slipped stitch from left to right, to pass the slip stitch over the k1 and off the right needle.