Sometimes you will see instructions in a knitting pattern to psso or pass slipped stitch over, as a method of decreasing a stitch.
It's commonly seen with another decrease such as in this knitting code: sl 1, k2tog, psso. In English that means slip one stitch, knit the next two stitches together, then pass the slipped stitch over the stitch you just made by knitting two together.
The pass slipped stitch over is performed in the same way you slip one stitch over another when you are binding off.
When you slip a stitch before a psso, it should be slipped purlwise, meaning from back to front as if you were making a purl stitch, unless otherwise noted in the pattern.