If an object with a higher temperature comes in contact with an object of lower temperature, it will transfer heat to the lower temperature object. The objects will approach the same temperature and in the absence of loss to other objects, they will maintain a single constant temperature. Therefore, thermal equilibrium is attained.
The Thermodynamics Zeroth Law states that if two systems are at the same time in thermal equilibrium with a third system, they are in equilibrium with each other.
If objects A and C are in thermal equilibrium with B, then object A is in thermal equilibrium with object C. Practically this means all three objects are at the same temperature and it forms the basis for comparison of temperatures.
The Zeroth Law of Thermodynamics states that:-
two system which are equal in temperature to a third system are equal in temperature to each other.