The "zeroth law" states that if two systems are in thermal equilibrium with a third system, then they must be in thermal equilibrium with each other.
This law states that if object A is in thermal equilibrium with object B, and object B is in thermal equilibrium with object C, then object C is also in thermal equilibrium with object A.
The message of the zeroth law is: “Every body has a property called temperature”. Two objects are defined to have the same temperature if they are in thermal equilibrium with each other. This law allows us to build thermometers. For example the length of a mercury column (object B) may be used as a measure to compare the temperatures of the two other objects.
The zeroth law, which has been called a logical afterthought, came to light only in 1930s, long after the first and second laws of thermodynamics had been discovered and numbered. Because the concept of temperature is fundamental to those two laws, the law that establishes temperature as a valid concept should have the lowest number-hence the zero.
The zeroth law has a fairly straightforward statistical interpretation and this will allow us to begin to establish the equivalence between the statistical definitions and the conventional thermodynamic ones.