The initials SSK in knitting stand for "slip slip knit." SSK is a basic method used in knitting that decreases the stitches, slanting them to the left. This method is often paired with "knit two together," which is a way to decrease stitches slanting them to the right.
To achieve the SSK decrease, the first stitch is slipped as if doing a regular knit, then the processed is repeated for the second stitch. Next, the left-hand needle slides to the front of both stitches, and they are knitted together. The "knit two together" method is executed by making a regular knit stitch, but instead of working through one stitch, it is achieved by going through two stitches.