Firstly AH is actually meant to be RH. Some digital display screens don't get it quite right. So, RH stands for Relative Humidity which is an indication of how moist or dry the air in your room is. A good level is around 50% and its good to keep the RH between 40% and 60%.
The "Mist" button has three levels of production which change each time you push the button. So you can increase or decrease the flow rate to achieve the RH % you want.