Make 1 Purl Stitch Left (m1pL)
The m1pL, or make 1 purl stitch left, is a left-leaning single increase (when viewed from the knit side of the fabric) that creates a new stitch from the space between two existing stitches.
To work it, use the left-hand needle to pick up the bar between the most recent stitch on the right-hand needle and the next stitch on the left-hand needle from front to back. Purl this new stitch through the back loop.
This article includes diagrams, and the following video demonstrates how to work a m1pL around the 2:54 mark: