SSK is one way of decreasing stitches. SSK means slip slip knit. So you slip the next stitch off your needle as if to knit, then slip the next stitch off the needle as if to knit and then you insert the left needle into the front loops of the slipped stitches and knit them together from this position (through the back loops)
You can see how to do this in Amy's video: SSK
Hope this helps!