What is epiphenomenalism?

Epiphenomenalism is a philosophy based on the principle that mental activity—thought, consciousness, feeling, and so on—is an "epiphenomenon," or a byproduct, of physical activity. All the functions of the physical body, like muscles contracting and nerves firing, are carried out without any mental effort. Our perception of mental effort, decision-making, or consciousness is just an aftereffect of the physical body doing all the actual physical work.
For example, imagine a situation where you feel hungry and decide to eat. Your perception of this situation leads you to believe that your mental activity is determining the actions of your physical body: you mentally "feel" you are hungry and mentally "decide" to eat and mentally "tell" your body to get some food from the kitchen. According to epiphenomenalism, all this mental activity has no actual influence on what your physical body does in this situation. You could have no deliberate conscious thought and still feed yourself, driven only by physical reactions. Your perception of your consciousness "telling" your body to feed itself is not the real cause of your body eating food. Rather, this mental activity is an epiphenomenon—a kind of aftereffect of the regular functions of your physical body.