Bodybuilding (Merriam-Webster) is “developing the body through exercise and diet.” Specifically, one creates an environment conducive to promoting muscular hypertrophy (cellular growth) through the way they exercise and the food they eat. Contracting a muscle against a resistance greater than the muscle has adapted to is what produces the physiological change. The diet portion is crucial to bodybuilding as it dictates how much energy your body can spend on growing, thus forming new muscle fibers or ridding itself of body-fat. In bodybuilding, the aim is to isolate individual muscles with each exercise, rather than using several muscle groups synergistically. For example, a bench press for bodybuilding purposes seeks to contract the chest as much as possible, and use the deltoids as litte as possible. This is opposed to trying to lift as much weight as possible, as in weightlifting or powerlifting. The dynamics are entirely different! Bodybuilding seeks to use ONE muscle in its entirety, rather than using mechanical advantages and synergistic muscle groups to move more weight. 

The human body simply adapts to the stimulus it is presented with. The more often it does something, the more efficient it becomes at that specific thing. The more you contract a muscle against a resistance, the easier it becomes to contract against that specific resistance. This is why we must increase the resistance to initiate growth. Conversely, decreasing the resistance will cause the body to adapt to the lighter weight, eliminating its need for anything unnecessary to move the weight (excess muscle mass, for example).