This means that whenever system A is in thermal equilibrium with system B, and system B is in thermal equilibrium with system C, then system A and system C are also in thermal equilibrium.
Wann e System A mat engem System B am Gläichgewiicht ass, an de System B mat engem drëtte System C am Gläichgewiicht ass, dann sinn och de System A an C am Gläichgewiicht.