Every chemical reaction involves breaking bonds in the reactants and making new bonds in the products. Breaking bonds is endothermic: it takes in heat energy. Making bonds is exothermic: it releases heat energy.
The overall energy change depends on which of these two is bigger. If more heat energy is released in making the bonds in the products than is needed to break the bonds in the reactants, the reaction is exothermic overall. The surroundings get hotter.
If less heat energy is released in making the bonds in the products than is needed to break the bonds in the reactants, the reaction is endothermic overall. The surroundings get colder.
