M1 stands for "make one" in knitting. This is a generic phrase that indicates that you should increase the stitches. M1 is also known as an invisible increase.
There are several way to do an invisible increase. If the pastern does not specify there is a generic version. Make a small hole with a horizontal strand of yarn running between the two stitches in the row below, and then put the right needle into that strand going from front to back. Move the strand onto the left needle. Knit in the back of the strand on the left needle as normal and then remove normally.