M1 is the abbreviation for a make one. A make one creates a stitch by picking up the bar of yarn stretching between two stitches, and knitting it through the back loop.
How to work a M1
Going from back to front, with the tip of the right needle, pick up bar running between the stitch on the right needle and the stitch on the left needle. Bring bar up onto left needle. Knit into back leg of loop created by bar.