Just as two systems in mechanical equilibrium have the same pressure, two systems in thermal equilibrium have the same temperature.
Consider two systems A and B in thermal equilibrium with each other (separated by a thermally conductive wall). Similarly, let us assume that B and C are also in thermal equilibrium. It is logical that A and B are at the same temperature and also B and C, but what is the relationship between the temperature of A and C? It is observed experimentally when putting systems A and C in contact through a thermally conductive wall that no heat flow occurs, that is, A and C are also at the same temperature.
This experimental fact is known as the zero principle of thermodynamics: “Two systems are in thermal equilibrium with a third one with each other”.