SKP is the abbreviation for slip, knit, pass slipped stitch over. This is a single decrease and can be used in place of the SSK. In knitting tradition, the SKP is an older, earlier version of the SSK, so the SSK is more common in modern knitting patterns.
Like the SSK, this is a single, left leaning decrease. Meaning, it combines two stitches into one, and leans to the left.