The zeroth law of thermodynamics is a very basic and useful law. For example, If we want to know whether two objects are at the same temperature, we measure the temperature of each with a thermometer.  We do not need to bring the two objects in contact to determine whether or not they are in thermal equilibrium.

The zeroth law of thermodynamics states this experimental fact as follows:
If object A is in thermal equilibrium with object B and object B is in thermal equilibrium with object C, then object A is in thermal equilibrium with object C.

It is an experimental fact that the temperature of an object alone determines whether or not the object is in thermal equilibrium with other objects. It is only the temperature that determines whether or not there is heat flow from or to an object from its surroundings.