The zeroth law of thermodynamics establishes a reference temperature for the measurement of temperature. It defines that if two objects or systems are each in thermal equilibrium with a third system, then the first two objects are in thermal equilibrium with each other.
Imagine first calibrating a thermometer with a calibration pattern. Then, because you have the thermometer scale based on the pattern, you can use it as a reference for measuring a third object, fulfilling the zeroth law statement. Read more