Complex sentence: A punching bag is a sturdy bag designed to be repeatedly punched, for use in physical exercise or stress relief in order to improve one of three areas: physical strength, aerobic fitness, or punching technique.
Split into simpler sentences: A punching bag is a sturdy bag designed to be repeatedly punched, for use in exercise, physical training, or stress relief. Someone who trains using a punching bag is usually trying to improve one of three areas: physical strength, aerobic fitness, or punching technique.