The principle of progressive overload in strength training involves gradually increasing the stress placed on the muscles to promote growth and strength gains. This can be achieved by lifting heavier weights, increasing repetitions, or adjusting workout volume and intensity over time. By consistently challenging the muscles beyond their current capabilities, adaptations occur, leading to strength improvements and muscle growth. This principle is crucial for anyone looking to maximize their strength training results safely and effectively, as it prevents plateaus and fosters continuous progress toward fitness goals.