An exothermic reaction is one in which energy is transferred to the surroundings, usually as heat. The temperature of the surroundings rises. A heat pack is a simple example. When it is activated, it gets warm and passes energy to your hands.
Cellular respiration is also an exothermic reaction. Glucose is broken down and energy is released. Some of this energy is used to make ATP and the rest transfers to the surroundings as heat.
This is why your body stays warm, and why a compost heap or a pile of germinating seeds feels warm to the touch. The living cells inside respire, and respiration releases heat.