The molel displays the change in entropy when changing the water state of aggregation by temperature increase. The entropy definition
implies that if heat flux is the same while the reversible transition of a system from one state to anoter, the entropy variations are swifter for lower absolute temperature T. Here at constant temperature, that corresponds to a two-phase state of the system (the level sections of the graph t(τ)), the entropy behaves linearly. The entropy increments in these sections are due to the change in the system phase composition.
On the left of the model you can see the diagrammatic representation of the system processes. The temperature being raised, the melting of ice starts (here ice is represented with the light tone and above water). Then heating of water to the boiling point occurs. The temperature of the water-steam two-phase system remains unaffected when boiling at constant pressure. After the whole of water becomes a steam the increase of gas (steam) temperature starts.