purl 1 stitch front and back
Pfb or p1fb simply means purl stitch front and back. This is an increase stitch.
To work this stitch simply work a purl stitch like you would normally, except you don’t remove the old stitch from your left needle. With your right needle come into that stitch your still working from left to right, this can be a bit fiddly the first few times, and then purl that stitch as you would normally, letting the old stitch go off your left needle.