The popular exercise activity that focuses on intervals of rest and exertion is called High-Intensity Interval Training (HIIT). HIIT alternates short bursts of intense activity with periods of rest or lower-intensity exercise, allowing individuals to maximize calorie burn in a shorter time frame. This training method can improve cardiovascular fitness, speed up metabolism, and enhance overall endurance. HIIT workouts are versatile, catering to different fitness levels, and can be easily adapted for various exercises, from running to strength training, making them an effective fitness choice.