Slipping a stitch means that you do NOT run the yarn through the stitch. Instead, you take your right needle put it through the stitch on your left needle either knit or purlwise, i.e. front to back with yarn behind your work or top to bottom with yarn in front of your work, and then simply take it off the left needle. Done!

Slipping stitches is often done when a tighter loop pattern is desirable. That might be the case when you work on the neck or armhole of a sweater.