In a chemical reaction, the bonds in the reactants must be broken and new bonds must be made to form the products. Breaking bonds needs energy to be taken in from the surroundings, so bond breaking is endothermic.
Making bonds gives out energy to the surroundings, so bond making is exothermic. The more strongly atoms are held together, the more energy is needed to break the bond and the more energy is released when it is made.
Every reaction involves both bond breaking and bond making, and the two have opposite energy changes. The overall energy change in a reaction depends on the balance between these two steps.
