Make ones are some of the most common increases. They are made by picking up the yarn in between two stitches, twisting it into a new "stitch", then knitting it.
There are two make one increases, make one right and make one left. If a pattern says to "make one" or "m1" without indicating "left" or "right," you can choose either one (but it's probably a good idea to keep it consistent).
Although it sounds like each increase should strictly go on the right or left sides of a project, make one rights and make one lefts produce only subtly different results when used on the opposite sides as their names. So sometimes you might be making a make one right on the left edge of a project, and vice versa.
This tutorial is for making make ones on the knitside of a project; if you need to make one on the purlside of any project, follow this tutorial.
m1 - make one
m1l - make one left
m1r - make one right