This stitch is a left-leaning decrease. It is usually abbreviated SSK.
Insert the right needle into the next stitch as if to knit, but slip it instead; and do likewise with the next stitch. Insert the left needle into these two slipped stitches as if you were transferring them back to the left needle, but once both needles are through both stitches, knit them together instead. The stitch is now complete; let both old stitches fall off the left needle.
This results in the same stitch as slip, knit, passover (SKP), so use whichever method of these two you prefer when either is called for.