SSSK is a left leaning double decrease. In other words, it decreases two stitches at once and the result leans to the left.
There are several different double decreases, left leaning, right leaning and centred. You may have come across them in lace and open patterns, not so often in shaping ordinary garments.
Here is a quick photo tutorial on how to work the sssk decrease. I hope you find it useful.