An endothermic reaction takes more energy to break the bonds of the reactants than is released when the bonds of the products are formed.
Chemical bond formation is always an exothermic process and chemical bond breaking is an endothermic process. When bonds are formed the system loses energy and hence increases its stability.
In this case; Cl + Cl —> Cl2 + 58 kcal ; bond between two Cl atoms are being formed and thus the reaction is exothermic as it releases energy to the surrounding.
