In my book, it is written that Hydrogenation of unsaturated hydrocarbons are exothermic reactions. Explain it in easy language.

UNsaturated Hydrocarbons are basically hydrocarbons with double and triple bonds in the chain. If we add 2 hydrogen atoms to either side of double bond then energy is released.Same goes when there is addition of 4 hydrogen atoms for triple bond.
The reason behind this exothermic nature of reaction or the reason behind the release of energy in this reaction because of the difference in the bond energy of the reactant and product side.