That's a left leaning decrease, the same as sl 1, k1, psso, only done a little differently. Look at the videos on the Decreases page.
Generally it's done at the beginning of a row to make the dec lean inward toward the middle of a section. But it can also be done at the end and make the dec lean toward the edge, and you'd do a k2tog on the other end.
sue- knitting heretic