The saturated hydrocarbons have single covalent bond between the C atoms.
This single bond is strong and it is not very easy to break this bond
The unsaturated hydrocarbons have double and triple bonds
Among these double and triple bond only one bond is strong i.e. it is not very easy to break one bond but the rest are very weak and can be broken down easily while taking part in a reaction to form different compounds
This implies unsaturated hydrocarbons are more reactive than saturated hydrocarbon (or) saturated hydrocarbons are less reactive then unsaturated hydrocarbon.