In order to achieve maximum benefit from your weight training, you'll need to train to failure.
So what's training to failure?
When you train to failure, your target muscles will no longer
be effective in the movement you are trying to train them in. For example, if you are doing barbell squats with a heavy
weight for many reps. After a certain number of sets, your quads will be severely worn and fatigued, leaving your legs all weak and you won't be able to perform any more reps. This is what training to failure is all about.
It represents a good benchmark of 'intensity' in your weight training. When you train your muscles to failure, you are essentially putting in 100% effort and so that your muscles can fully benefit from your training.
If you have the desire to build muscle fast and/or have quality/dense muscles, then you'll need to constantly train to failure. Some serious bodybuilders may even train past
failure to ensure that you fully utilise and wear out each and every muscle fibre. Training past failure involves finding a way to cheat and/or to get
help from elsewhere once you reach failure to complete your last few reps. This involves mild swinging of barbell/dumbbell for added momentum, using a friend to give a nudge, your other arm to help support and guide the movement etc. Training past failure ensures you train your muscles 110% for maximum result!
No pain, no gain!