The k2tog, or knit 2 together, is a right-leaning single decrease. It takes two stitches and turns them into a single stitch that slants toward the right.
To work it, insert your right-hand needle into the first two stitches on the left-hand needle at the same time, just like you would to knit a single stitch. Catch the working yarn with the tip of the right-hand needle and draw this new stitch through both of the stitches on the left-hand needle. Finally, slide those two stitches off the needle.
The following video demonstrates how to work a k2tog:
This video demonstrates how to work a ssk, but at 1:24, there’s a comparison of ssk and k2tog so you can see how they produce stitches that slant in different directions.