ssk = slip, slip, knit
Work to the place where the decrease is to be made. Slip the next 2 stitches, one at a time as if to knit, to the right needle.
Insert the left needle into the fronts of these two stitches and knit them together. One stitch is decreased. This decrease slants to the left.
On the purl side of the work, the decrease is called slip, slip, purl (ssp). In the same way as ssk, slip the 2 stitches on at a time as if to knit to the right needle. Purl them together through the back loops (see the FAQ on purling through the back loops for detail). This decrease slants to the left.