Lost in Translation

Ok, so as an intermediate knitter, I got a bit lost on a pattern a friend gave me before she went out of town… Searching online hasn’t found me any help in figureing out this stitch…
The pattern goes… k1, yo, *sl 1, k2tog, psso, … etc.
I get stuck on the psso (pass slip stitch over) and can’t find any more than that. WHAT do I do?

You slip one stitch, knit 2 together and pass the slipped stitch over the k2tog. It’s a double decrease.