Short for repetitions, high-rep training helps you stay in control of your strength training. When performing an exercise, like a bicep curl with a dumbbell, each time you bring the weight back down and up is one repetition. It takes more effort and energy to lift weights higher than these repetitions, so if your exercises are not spread out between sets, your body will use its fat-burning furnace on only one exercise for each set, resulting in less weight gain or muscle development. The same is true when performing decline bench presses - you can do fewer reps with a heavier weight but derive more benefits from the exercise because you're using more muscles to complete the movement.
Long for real, long-term strength training (LTR) requires that you regularly perform the same exercise through several sets. The reasoning behind this concept is that your body will adapt to a fixed number of reps, so if you stick to that number, over time your body will learn to anticipate the workout and do just as many For instance, if you're using a three-week program and performing five sets per week, you should expect to increase your LTR strength by two reps per set. For a bodybuilder, this translates to one rep per workout, six exercises per week. Most athletes are limited by their ability to perform long-term training (i.e. a career in professional baseball), but many bodybuilders still perform long-term strength training.
Exercises that induce high training volume (i.e., more sets performed in a smaller amount of time) will result in increased gains in muscle size and mass. Repetitions aren't everything; intensity is equally as important. To get the most out of your workouts, intensity (i.e., high volume) must be complemented with low volumes (i.e., low reps). So don't overdo it on your heavy day and underestimate your low volume day. Failure to do this will result in injury and will decrease the gains you're making.