In knitting language a purl, or 'P' is one of only 2 stitches on needs to know when creating.
When combined with one row purl and the other row Knit, or 'K', the lovely flat stocking stitch appears, a flat front to a knitted article with great stitch definition. In knitting, purl rows are important.
The Telegraph last weekend published an interview with Dr Thomas A. Ernst FRCP and myself about knitting and overcoming depression.
You can have a read here.