Purl two together through the back loop, or "p2tog tbl", makes a right-leaning decrease (and if you're working in stockinette, a left-leaning one on the knit side). It also sometimes called an "ssp", or "slip slip purl".
Here's how to do it!:
1. Slip the first 2 stitches knitwise from the left needle to the right needle.
2. Slip them back onto the left needle, maintaining their orientation. These 2 stitches are now on the left needle in a twisted position.
Reach your right needle around to the knit side of the work and insert the tip through the back of the second stitch and then through the first.
Now bring the tip of the right needle to the front of the work and purl together those two stitches.