It is also known as Hess's law of constant heat summation.
It is named after German Hess a Russian chemist and physician.
It states that “ whether a chemical reaction is conducted in a single step or in multiple steps, the enthalpy change will always be the same”.
The nature and state of the reactants and products determine the enthalpy of a reaction. It is unaffected by the path that the chemical reaction takes.
The law is actually only a specific case of the first law of thermodynamics (law of conservation of energy) i.e. energy can neither be created nor can be destroyed.