The principle of progressive overload in fitness training is fundamental to achieving physical improvements. It involves gradually increasing the demands placed on the body during exercise to enhance strength, endurance, and overall fitness. This can be achieved by adding weight, increasing repetitions, or adjusting the intensity of workouts. By consistently challenging the body, it adapts and grows stronger, thus preventing plateaus in training. It is essential, however, to implement this principle carefully to avoid injuries and ensure optimal recovery.