They want to know what number is in your head. And although strategies vary on this depending on the person dispensing advice, I’ve always gone with the theory of getting them to throw out the first number.
First and foremost, talking about salary before they’ve made you the job offer seems counterproductive and putting the cart in front of the horse. If at all possible, try to make sure they really want you first before you start negotiating salary. Once they put out the first number, assuming it’s in the acceptable ballpark, negotiate up from there while relying on your prior compensation research. And all the better if you have the offer because you know they want you and it strengthens your bargaining position.