Purling stitches together, as in p2tog, p3tog, etc, is a common purl decrease. P2tog is the purling equivalent of the k2tog.
P2tog leans to the right as the k2tog does. If working the decrease on the wrong side, a p2tog is usually used where a k2tog is used on the right side, and a p2tog-b or SSP is worked where an SSK, SKP, or k2tog-b is worked on the right side.
Purl 2 stitches together
Abbreviation: p2tog
As if to purl, put right needle into first two stitches on left needle, then purl as one stitch.
Purl 3 stitches together
Abbreviation: p3tog
As if to purl, put right needle into first three stitches on left needle, then purl as one stitch.
Purl 4 stitches together
Abbreviation: p4tog
As if to purl, put right needle into first four stitches on left needle, then purl as one stitch.
Purl 5 stitches together
Abbreviation: p5tog
As if to purl, put right needle into first five stitches on left needle, then purl as one stitch.
See also: Knitting Stitches Together, Knitting Stitches Together Through the Back Loops, Purling Stitches Together Through the Back Loops