purl 2 stitches together
P2tog means purl 2 together, or simply purl 2 stitches together.
It’s a common decrease stitch for the purl side of your work.
Start off like you normally would a purl stitch except this time your going to bring your right hand needle through 2 stitches on your left needle, not just the first. You’ll now purl as normal, bringing the yarn around, picking up the new stitch and slipping the old stitches off the left needle.