You have probably noticed that medications come with a particular dosage, or recommended amount to get the desired effect. If you take too little, nothing happens. Too much and you experience unpleasant side effects. Did you get your daily dose of coffee today? If not, you may feel tired or have a headache. In each case you provide your body with an external stimulus, and the effect of that stimulus depends on how much of it you receive.
Exercise also provides us with external stimuli, and if we want to achieve a desired effect, we need to choose the right "dose". If we want to get bigger and stronger, than lifting weights is a great way to do so. When performing particular resistance training exercises, the weight used/reps/sets/rest time are variables that determine the dose of the movement. In order to improve strength and size, we have to progressively change these variables so that they offer a greater and greater stimulus. When we undergo these progressive variable changes, we are building a training program.
The best training programs constantly make each workout progressively harder. This happens when the intensity increases, the rest time goes down, etc. These changes stimulate the body to progressively adapt to these "tougher workouts." This is what leads to results. When evaluating a training program, check to see if these variables change progressively and consistently. If they do, and the "dose" that it creates accurately fits the fitness level of the person doing it, improvements in performance should follow.