ROM stands for Range of Motion. It tells you and your Clinician how much movement you have at your knee joint.
Often times, one of your primary recovery goals will be to improve ROM. For example: Your Physician or Physical Therapist may say your goal is to increase knee ROM to 0-120. This would mean your goal is 0 (fully straight) to 120 degrees bent or flexed which should allow you to complete most functional activities, such as descending stairs or squatting, without compensation.