It means getting enough sleep to allow for recovery and basic health. It means always trying to get stronger. It means following Bob Whelan's Ten Commandments. Intensity is the most important variable of a cardiovascular or weight training program. Your success in changing your body, specifically your muscles, will be determined by whether or not you lift with high intensity.
High intensity means working to momentary muscular failure.
That means you cannot perform another rep, with proper form. If you always lift the same amount of weight for the same number of repetitions, your body can obviously handle the stress. Therefore, it will make no changes since it doesn't need to change. If you increase the demands placed on your body, it will respond in turn. As you increase the amount of weight, you force the muscles to develop strength and eventually some size.