Every cell in your body carries out metabolic reactions, and each one is controlled by an enzyme. Enzymes work best at an optimum temperature and pH. If conditions drift away from the optimum, the reactions slow down, and a large change can make an enzyme become denatured so that it stops working altogether.
Homeostasis is the regulation of the internal environment of the body. It keeps conditions such as temperature, blood sugar level and water content steady, even when the surroundings or your activity change. A receptor detects a change, a coordination centre processes the information, and an effector brings about a response that returns the condition to normal.
The body has to cope with internal change, for example blood sugar rising after a meal or temperature rising during exercise. It also has to cope with external change, for example a cold day cooling the skin. Keeping the internal environment constant allows metabolic reactions to proceed at appropriate rates.