The Complete Strength workout includes a column for intensity. This element is designed to tell you how hard the exercise should be in terms of your maximum ability to lift your weight. Since the goal of the workout is that you should lift with perfect form for every repetition, we want you to save at least one perfect rep in each set. So, intensity is always measured in a negative number, in terms of how near you come to your absolute exertion.
When you see a -1 in the workouts, it means that you should be one repetition away from a loss of form, or what weightlifters call "failure." Say you are doing a heavy dumbbell press exercise, and you get to the seventh repetition with perfect form, but you're really struggling and just barely make it. Well, stop there instead of trying to do another rep, and you're at a -1—ideal for that exercise.
For some exercises, you will see an intensity of -2, meaning that you could squeeze out two more correct repetitions before reaching failure. For these exercises, save 2 perfect form reps in you when you stop the set..
The point is never to get to the point where form breaks down. For any single rep where you allow your form to break, your body builds a faulty motor pattern, which can quickly lead to muscle imbalances and consequent injury. Our goal is to avoid this path. You will find over time that the amount you can lift while getting to a -1 or -2 in a set of the same exercise will increase rapidly—and indeed, that is the intent, to build strength over time.