Hello, Jen,
If a reaction starts with 100kJ, that is the potential energy stored in the various bonds of the reactants. Each bond has a defined amount of energy that can be released in an exothermic chemical reaction. Exothermic means the release of energy. In this case the release was 80 kJ, given off in forms such as heat, light, and sound. The 20kJ that were not released have found a new home in the bonds of the products. There it resides as potential energy for other reactions that might release it.
Bob
