Our bodies maintain homeostasis, a steady state or balance between the internal environment and external surroundings. We are endothermic. That implies that we generate our own heat that is stable, even when the outside temperature rises or falls. Ectotherms like fish, amphibians and reptiles cannot do this and rely on the environment for the heat they need to live. If it gets too warm, an ectotherm must seek shelter to survive. Heat is transferred from a warmer to a cooler object. If a human is cold, it means that if their body is 98.6 degrees Fahrenheit, and if the air temperature is 20 degrees, their body would begin to lose heat to the environment. At cold temperatures, sweat stops being produced. At first, goosebumps would appear, and this helps to keep heat inside the body. The circulating blood would be diverted to the inner organs and the fingers and toes would begin to feel numb. Shivering would occur. Mitochondria, in cells convert fat to heat energy to increase the cell's temperature. These are all mechanisms to try to maintain homeostasis. After awhile, if the person does not find shelter from the cold, they might get hypothermia and die.
Our body is at a certain temperature. When the temperature around us is colder our body slowly loses heat and we get cold. The same goes for heat but the other way around.