The ssk, or slip-slip-knit, is a left-leaning single decrease. It takes two stitches and turns them into a single stitch that slants toward the left.
To work a ssk, slip the first stitch on the left-hand needle as if to knit, then slip the second stitch on the left-hand needle as if to knit. Slide the left-hand needle into these two stitches from left to right and knit them together.
The following video demonstrates how to work a ssk:
At 1:24, there’s a comparison of ssk and k2tog so you can see how they produce stitches that slant in different directions.