The zeroth law of thermodynamics is a fundamental principle that establishes a basis for temperature measurement. It states that if two systems, say A and B, are each in thermal equilibrium with a third system C, then A and B are also in thermal equilibrium with each other. This means they share the same temperature. This law is crucial because it allows us to define temperature in a consistent way. By using a thermometer, which can be considered as the third system, we can measure the temperature of other systems and compare them, ensuring that our understanding of temperature is reliable and standardized.