"The skin is the body's largest organ."
When we think of our body's organs, we generally immediately think of the usual ones that do the work to keep us alive and functioning such as the heart, brain, liver, kidneys, etc. However the skin is indeed one of those organs, and due to the fact that it covers the entire body, makes it the largest organ we have.
The skin is part of the body's integumentary system. This means it is responsible for protection of the body. It provides protection for things such as temperature, bacteria, and other external factors. Like other organs it is made up of several layers containing different forms of tissue, and has multiple parts to it (also known as derivatives) such as specialized nerve endings, oil glands, sweat glands, hair and hair follicles, and even nails. It's main job is to create a protective barrier for the internal organs, and other systems.
In a human, the skin has been a functioning organ since the very beginning of their existence. At conception, one of the first things a fertilized egg does is splits into 3 parts: The Endoderm becomes the internal organs, the Mesoderm becomes the bones and muscles, and the Ectoderm becomes the brain and skin. So the skin is very closely tied to the brain and therefore everything that goes on there, from our thoughts and feelings, to how the rest of the body functions - it is all tied into the skin. This is one of the reasons that massage has been researched, studied, and used for centuries on infants. It has been shown that infants who receive massage gain weight faster, and can develop motor skills, coordination, and even pick things up faster than infants who receive little to no touch.
Many people jump at an opportunity for something that makes their organs healthier; exercise for your heart? Sure. A supplement for your brain? No problem. Drink extra water for your kidneys? Easy. So even though there are many people who still consider massage as just a luxury that has no medicinal value, this couldn't be farther from the truth. While massage can be considered a nice treat, since it deals greatly with manipulation and care of the skin, it can easily be classified as care, exercise, and improving the health of a very important and large organ, which in turn can improve the health of everything the skin protects.