ADS means "Aiming Down Sights." It's not really accurate to say it means "using the scope," but in the context of your question, that's the answer -- it means using the scope. Really, ADS refers to what happens when you pull the left trigger in most modern shooters, including destiny, where the gun is brought up close to "your screen" to let you see down the sights of the gun (whether or not the sights include a scope) to see exactly where your shot will land.