The M1L (which stands for "Make One Left") is a left-leaning increase stitch. It is worked into the horizontal strand of yarn (or "bar") between the first stitch on the left needle and the last stitch on the right. For a right-leaning increase, see M1R.
The make-one increases can be a bit tricky to get the hang of. It’s often difficult to insert the right needle through the back leg of the strand, since it’s not a true "loop."