“Make 1” is an almost invisible way of making a new stitch by picking up the thread that lies between stitches, twisting it, and knitting or purling it.
Left slanting M1
Place the thread that lies between your stitches onto your left hand needle with the right hand side of the loop at the front of the work.
Knit (or purl) into the back of the stitch.
Right slanting M1
Place the thread that lies between your stitches onto your left hand needle with the left hand side of the loop at the front of the work
Knit or purl into the front of the loop
If you forget to twist the picked up loop before knitting (or purling) it, you will create a hole in your work. If you don’t want to create a hole this can be a problem.
In antique patterns the term “Make 1” was often used instead of yarn over. If you’re working from an antique lace knitting pattern and it contains a large number of Make 1’s and no yarn overs, you can be reasonably sure that you should be working a yarn over instead.