Make One (M1) Increases
A Left-Leaning or Right-Leaning Single Increase
'Increases' are methods used to increase the overall number of stitches on your needles. This allows you to control the width of your knitting and add shape.
The 'Make One' (M1) is a single increase, which means that it increases the total number of stitches on your needles by 1.
Unlike most other increases (and decreases), you can choose whether to make your M1 increase left-leaning (M1L) or right-leaning (M1R).
When I say 'right-leaning' or 'left-leaning', this just refers to how the increase will look from the front of your knitted work; they will either lean slightly to the right or to the left.
A pattern will usually tell you which one to use, but if it doesn't I would recommend choosing the M1L by default. But whichever one you do choose, stick with it throughout your project so that you get consistent results.
The additional stitch formed by an M1 increase is created from the horizontal strand of yarn that runs between your needles. Because of this, you can do a 'make one' at any point along a row, except as the very first stitch on a row.
The M1 is a popular increase, and I think the main reason for this is that, unlike the kfb for instance, it doesn't create an obvious 'bump' or other marker on your knitting.
It is often referred to as an invisible increase.
Please find the video tutorial below:
I hope this is clear and useful for you :)