The zeroth law of thermodynamics says that two objects in thermal equilibrium with a third object re also in equilibrium with each other.
The zeroth law of thermodynamics is called the zeroth law and not the fourth law because it is more fundamental than the first law but it was discovered after the other three.
Imagine having three interconnected beakers of water so that the water levels in all three are the same height. They are interconnected so that any change in one beaker gets balanced out so all three beakers end up ay the same level.
The reason why this occurs is that heat can flow between all three objects so that if one gets out of equilibrium the others supply heat to it or removed heat from it there by restoring equilibrium to all three.
The zeroth law is a rather simple concept and it is the most fundamental of the Laws of Thermodynamics.