So what does that have to do when applied to weight training? I am asked how many reps and set should I be doing? That all depends on your goals. One who is interested in fitness, and just being in shape wouldnt neccisarially do heavy weight with low reps. So in order to decide you need to set a goal.
Repetitions between 3-8 are used mainly for adding strength. Repetitions between 8-12 is where Hypertrophy comes in. Training in hypertrophy is where you will see the most results in regards to growth. So if you are wanting to add muslce, you should train in the 8-12 rep range. Repetitions between 12-15 are used more for beginers,and those just training to maintain what they might already have.
Now once you decide what your goals are, You will have to decide what weight you should be lifting. If training for size, and you are in the 8-12 rep range, when you get to the 10th repetition, and its still easy,then its to light. You should be struggling to get that last repetiton. If you are, then you are doing it right!
Whatever you are training for, whether it be size, strength or just health and fitness, make sure you push youself. The body adapts very quickly to what we throw at it. So if you are looking to chage your body, push it!