Lady Capulet relates Juliet's understanding to that of someone who still does not have requisite understanding due to her age. Her statement, that there is little more useless Than a boy means that indulging in a relationship is almost the last thing one should adore or get preoccupied with. Therefore, we can deduce that Lady Capulet gives significantly more credence to duty than love. Hence, she feels Juliet's who has given in to love is mistaken and ties Juliet's understanding to her lack of experience.
I believe the answer is a stage whisper. A stage whisper is defined as “a loud whisper uttered by an actor on stage, intended to be heard by the audience but supposedly unheard by other characters in the play.”
