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

Step 1 – Insert the left hand needle from front to back into the horizontal strand between the stitches on each needle.

Step 2

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

Step 3 – Pull yarn through, dropping the stitch of the left hand needle.

 One increase completed.