pass slipped stitch over
Psso simply means to ‘pass slipped stitch over’. This will always be used in conjunction with a slipped stitch instruction.
An example of a psso in use would be sl1, k1, psso. So you’d knit as slip 1 stitch as if to knit, then knit one stitch. Now you’re going to do the ‘pass slip stitch over’ part, like this.