The Third Law of Thermodynamics states that:
The entropy of a perfect crystal is zero when the temperature of the crystal is equal to absolute zero (0 K)
In this state: The molecules within it move freely and have high entropy. If one decreases the temperature below 100°C, the steam gets converted to water, where the movement of molecules is restricted, decreasing the entropy of water. When water is further cooled below 0°C, it gets converted to solid ice. In this state, the movement of molecules is further restricted and the entropy of the system reduces more. As the temperature of the ice further reduces, the movement of the molecules in them are restricted more and the entropy of the substance goes on decreasing. When the ice is cooled to absolute zero, ideally the entropy should be zero. But in reality, it is impossible to cool any substance to zero.