Essentially an exothermic reaction is any reaction which gives off heat

Exo (to exit) and thermic (heat)

These reactions occur constantly throughout nature, as you can see from the video above they can be as big as a nuclear blast or as small as a glucose molecule being broken down in the aerobic respiration, however we call this an exergonic reaction.

You will know that sugar is a relatively simple molecule and easily broken down by the body.

This is shown in the reaction to the right. Each molecule of glucose is being broken down, and the energy released is lost as heat.

Potassium chlorate + gummy bear.

In an exothermic reaction the reactants contain more energy than the products.

Notice as the gummy burns the energy level of the actual gummy decreases.
This is because a large amount of energy is released as heat.

The equation is bellow

8 KCLO3 (potassium chlorate) + C12H22O11 (Gummy bear)

12 CO2 (g) + 11 H2O (g)+ 8 KCL (potassium chloride)

As you can see from the equation above the products have significantly less energy than the original gummy bear.