Progressive Overload
Making consistent progress through incremental increases in stress (more reps, more challenging exercises/workouts, etc.) planned out correctly to allow your body to continue to improve without setbacks.
To simplify, we want to build your strength and fitness from where you are currently and then build slowly and intensionally to provide you the correct amount of stress and strain so that your body continues to get stronger, leaner, and more resilient without feeling negative effects of over working.
Too often we see people either pushing too hard and not recovering correctly. This leads to frustration because they're not seeing or feeling the results of their hard work. OR we see people who do not work hard enough to effect any change.