How to Knit - Increasing Between a Stitch ('Make One')
M1 = make 1
This increase is also called 'Make One'. Work to the place where the increase is to be made. Insert the left needle from front to back into the horizontal strand between the two stitches:
Knit the stitch through the back loop as shown.
On the purl side, insert the needle from front to back and then purl the stitch through the back loop.