Energy is transferred in every reversible reaction, and the changes in the two directions are linked. If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction. An exothermic reaction releases energy to the surroundings. An endothermic reaction takes in energy from the surroundings.
The same amount of energy is transferred in each case. If the forward reaction releases 60 kJ, the reverse reaction takes in 60 kJ. Energy is not lost or created, it is just given out one way and taken in the other.
Hydrated copper sulfate shows this clearly. Heating the blue solid to drive off water is endothermic, because it takes in energy. Adding water to white anhydrous copper sulfate is the reverse reaction, so it is exothermic and the mixture gets warm.
The thermal decomposition of ammonium chloride is also endothermic. The reverse reaction, where ammonia and hydrogen chloride combine, is therefore exothermic.