M1 (make one) is an increase stitch worked between 2 stitches. This stitch increases the total number of stitches on your needle by one.
How to work a M1 increase
So I have cast on 10 stitches and worked a few rows in stocking stitch, on the next knit row the instructions are :
K4, M1, knit to end. (11 sts)
I have knit the first 4 stitches of the row and am now ready to work M1:
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, wrap yarn around (as you would with a normal knit stitch).
Step 3 – Pull yarn through, dropping the stitch of the left hand needle.
One increase completed.