In practical terms, this means that if you train with the same weight every day, your body adjusts and compensates. Thuth is, in weight training, the only way you can grow muscle is to put your body in an overload condition. When you hit a plateau, your muscles essentially stop growing. It is imperative that you substitute your old routine with a more difficult one. It depends on your goals.
Muscular endurance comes from higher reps and high intensity.
Muscular size and strength comes from higher weights, higher intensity and fewer reps. If you're new to the muscle game and you want to experience muscular hypertrophy firsthand, aim for about 8 reps per set. Studies suggest that 8 strikes the perfect balance between size and definition. 3 to 5 sets of eight reps. I genarally don't recommend more than 8 reps for people trying to gain size.