m1pl (make one purl left) , is an increase worked on the purl side of the fabric between 2 stitches, forming a left slant when looked at on the knit side of the fabric. This stitch increases the total number of stitches on your needle by one.
How to work a m1pl increase
So I have cast on 10 stitches and worked a few rows in stocking stitch, on the next purl row the instructions are :
P4, m1pl, purl to end. (11 sts)
I have purled the first 4 stitches of the row and am now ready to work m1pl:
Step 1 – Insert the left hand needle from front to back into the horizontal strand between the stitches on each needle.
Step 2 – Insert the tip of the right hand needle into the back loop purlwise (from left to right), wrap yarn around (as you would with a normal purl stitch).
Step 3 – Pull yarn through, dropping the stitch of the left hand needle.
One increase completed.