The overload principle may be one of the most important concepts to understand as far as training is concerned. The human body is always changing according to the amount of physical stress, or lack there of.
Placing your body under stress includes lifting heavy weights, running, sprinting, or playing a sport. Overload is when you place a level of stress on your body that it is not used to. When overload occurs your body naturally adapts to that stress so that next time it experiences that stressor it can handle it better.
This is exactly why you must train according to your specific goal. If you want to get big strong muscles you have to stress those muscles with heavy weight so they grow and get stronger as your body adapts. If you want to be able to run long distances you must make your body gradually adapt to running farther and farther by overloading with long distance runs.
When you are out of shape your body is adapted to the lack of stress that is has experienced.