The p2tog, or purl 2 together, is a single decrease that leans to the right when viewed from the opposite side of the work. It takes two stitches and turns them into a single stitch that slants toward the right and is often used on wrong-side rows.
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 purl 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 p2tog: