The Five Element theory is an ancient Chinese philosophical concept that was used to explain the composition and phenomena in the universe. The Five elements are wood, metal, water, fire and earth. They are analogies in Chinese medicine to represent the five major organ systems. (There are actually six Yin and six Yang organ systems.) Wood is liver, fire is heart, water is kidney, metal is lung and earth is spleen.
The Five Element theory is one of the most important concepts in TCM. In TCM literature, the five elements and organ systems are used interchangeably, i.e. wood and liver mean the same thing when discussed.
The five elements are interconnected in all possible ways. This also illustrates how interconnected the five organ systems are. The simplest way to view these complex relationships is that when issues with one organ system are not correct, over time, they will spread to other systems. For example, excess heat or yang in the liver will, over time, spread to the heart, which will manifest itself with symptoms such as restlessness, insomnia, anxiety, etc..
Five elements are also connected to seasons, emotions, time of day, tastes and colours.