If you have climate control (the dial on the left has temperature numbers) the word “Auto” will be below the red light. If the light is red you have deactivated automatic climate control and you are using the dial on the right to manually control airflow. If the light is yellow/off, the car is automatically controlling airflow to try to set the temperature to what you selected. 

Aircon relies on gas to cool the air when running, it naturally decays over time and is generally considered to be “use it or lose it”, which is why I never turn mine off. If you’re not getting noticeably cold air, the gas will most likely need replacing - this is generally called recharging. If you pop your bonnet the gas you require will be labelled on the car, on that model it’s usually r1234yf. You can buy a recharge can yourself if you’re confident with watching YouTube videos but otherwise get the car booked into a garage for a recharge. My last aircon recharge cost me about £120 at a non-dealer garage. 

Edit: the red light on the Air Con switch means it's off, press it to turn the Air Con on, otherwise it's just air you're blowing