This document discusses the common mistake of doing cardio before weight training and how it can prevent fat loss and muscle gain. It introduces the "Hierarchy Scale of Energy" that the body uses during exercise, with muscle glycogen being burned first and fat being burned last. Doing cardio before weights means muscle glycogen is depleted, leading to a weaker weight training session. Instead, it recommends doing a short warm-up, then weight training to deplete other energy sources, leaving only fat available for a more effective post-weights cardio session.