It is a fact that the natural tendency of heat is to get transferred from high temperature source to low temperature source. But, in case we need to transfer from low to high temperature reservoir, what should be done? This problem is resolved through Clausius statement of second law of thermodynamics which states that reverse transfer of heat does not occur spontaneously.
The statement of second law of thermodynamics which is known as the Clausius statement says that it is not possible to create a device that can operate in a cycle and help to product no other effect rather than transferring heat from colder object to hotter body.
The statement further clarifies saying that transfer of heat from low to high temperature reservoir occurs with some external factor and it needs to be done on cycle. This statement turns out to be a basis that helps to perform refrigerator and heat pumps.