the complete directions (and doesn't your pattern have a KEY to abreviations?--did you read the key? (in knitting zines, its often on a seperate page.. (be they on line or printed zines))
the complete directions are S1, K1, PSSO
so you slip 1 stitch, Knit 1 stitch, then pass the slipped stitch over (the knit stitch)
is a decrease --specifically a left leaning decrease (vs a K2tog --right leaning decrease)
[COLOR=Red]NOTE 1[/COLOR]: a SSK (slip 1, slip 1, then knit these 2 stitches together (through the back loop) is also a right leaning decrease
[COLOR=Red][B]NOTE 2[/B][/COLOR]: if you are a combo knitter, you need another style of decrease for a right leaning decrease--google combo knitting for reams of info.