This decrease appears on the knit side of your work and is usually used on the right edge of the row. It's often abbreviated as "sl1, k1, psso" and sometimes as "SKP." Both stand for the same technique, which is "slip 1 stitch, k 1 stitch, then pass the slipped stitch over."
Slip one stitch as follows: With your yarn in back of the work, put the RHN from back to front into the first stitch on the LHN as if you were going to purl the stitch. Without doing anything with your yarn, carefully slip that first stitch from the LHN onto the RHN. Knit the next stitch.
Pass the slipped stitch over as follows: On the front of your work, put the LHN from left to right into the second stitch (the slipped stitch) on the RHN. Lift this stitch over the first stitch (the previous knitted stitch) on the RHN, and then over the tip of the RHN; let it drop completely off both needles. You have just decreased one stitch.