Wikipedia has an interesting explanation of Hess Law.
Hess;_Law

Basically Hess Law states that the measurement of the energy content of a system (called enthalpy) change in a reaction is equal to the sum of the enthalpy changes of the individual steps of the process. If you measure the energy content of a system change, it will be the same either it takes place in one big step or in many small ones. We can say in different words that if you go from a particular set of reactants to a particular set of products, the enthalpy change is the same, whether the reaction takes place in one step or a series of steps. This means that the enthalpy is a state function.