How to Slip, Slip, Knit
Slip, slip, knit (ssk) is a common form of decreasing in knitting. It leans to the left, while the knit 2 together (k2tog) decrease leans to the right. Designers will specify which decrease to use, since the different directions can be design elements in the pattern.
Slip the next stitch knitwise to the right needle.
Slip the next stitch knitwise to the right needle. You now have two unworked stitches on your right needle.
Insert the point of the left needle through the front of both unworked stitches.
Knit these two stitches together through the back loop.