SSK is a type of decrease. You're knitting two stitches together. Normally when you knit two together, you will insert the tip of the needle into two stitches at once in the front of the needles from left to right. With an SSK, you'll insert the needle into both stitches in the back of the stitches from right to left, then draw the yarn through both loops.
What's the difference? In a regular knit two together, the stitches will lean to the right. In an SSK, the stitches will lean to the left. You will understand this when you knit a pair of mittens. It leaves a nice line in the round shaping at the top. Here's what it would look like on mitten shaping:
K2together //// \\\\SSK.
left side right side
of mitten of mitten
You knit the type of decreases the pattern calls for in the places it specifies, then do the slip stitches on all the other stitches.