SKP (slip one stitch, knit one stitch, pass slipped stitch over) is a left-leaning decrease in knitting. It is also can shown in patterns like this:
sl 1, k 1, psso (psso meaning "pass slipped stitch over")
To work the SKP in your project, knit to the area in which you'd like to work the stitch,
Next, slip the stitch as if to knit,
Now pass the slipped stitch over with your left needle. This step is similar to when you bind off your knitting project.
All finished! Here's what it will look like when you are done: