Complex sentence: A punching bag (US) or punch-bag (UK) 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 (US) or punch-bag (UK) is a sturdy bag designed to be repeatedly punched. Athletically, it's used to improve one of three areas: physical strength, aerobic fitness, or punching technique.