The answer is quite simple. The heavier the weight and the more your body will have to work to move that weight. However, be sure when lifting weights to lift the appropriate amount of weight to avoid injury. Also, be sure if lifting heavier than normal to have a spotter.
When you lift heavier, your type II muscle fibers kick into gear. These fibers are fast-twitch and will help produce a more explosive motion. When you lift heavy, you will not be able to produce as many repetitions as if you were to lift lighter. When you lift light your type I muscle fibers kick into gear. These fibers help with building endurance in the muscle.