M1 means to make 1.
You can make 1 leaning to the left or make 1 leaning to the right. If the pattern doesn't specify which one to do, then you can do whichever you are most comfortable, just be consistent throughout the pattern.
If the pattern has you doing paired increases (i.e. an increase on either side of a marker/stitch/cable/pattern) then make the increase before the marker lean right and the increase after the marker lean left.