The pfb, or purl front & back, is a single increase that creates two stitches from one.
To work it, purl the first stitch on the left-hand needle, but DON’T take it off as you normally would! Instead, with the stitch still on the left-hand needle, bring your right-hand needle around to the back of the left-hand needle and purl into the back of the same stitch. Finally, slide the original stitch off the needle.
The following video demonstrates how to work a pfb. Both English and Continental styles are shown.