slip 1-k1-psso = slip 1, knit 1, pass slip stitch over
slip1-k1-psso = slip 1, knit 1, pass slip stitch over is a left slanting decrease.
Slip one stitch as if to knit.
Knit the next stitch.
Insert left needle into front of slipped stitch on right needle.
Pass the slipped stitch over the knit stitch and off the right needle.