Your body is always dealing with change. Some change is external, such as a hot day or a cold swim. Some is internal, such as the glucose that enters your blood after a meal or the water you lose when you sweat. Homeostasis is the way the body keeps conditions inside it constant, including body temperature, blood glucose level and water content.
This matters because of metabolic reactions. Metabolic reactions are the chemical reactions in cells that keep you alive, and they are controlled by enzymes. Enzymes work at an appropriate rate only in the right conditions. If the body gets too hot, enzymes lose their shape and denature, so reactions stop working properly. If the body gets too cold, as in hypothermia, reactions become slow and the body cannot function. Cells also need a steady supply of glucose for respiration and the right amount of water.
Control systems work by detecting a change and reversing it. Receptors detect the change, a coordination centre processes the information, and effectors produce a response that brings the condition back to normal. This is called negative feedback.