When heat is added or removed from the substance and the temperature stays the same: the Thermal Energy turns to Potential Energy.
The horizontal portions of the graph represents: Phase change and the temperature's constant state
How to Interpret the thermal graph?
According to the first law of thermodynamics, the amount of heat energy given to a system may be used for increasing the internal energy and for doing external work.
The internal energy of a system corresponds to all degrees of freedom like translational degrees of freedom, rotational degrees of freedom, and vibrational degrees of freedom.
Thus, we can say that when heat is added or removed from the substance and the temperature stays the same, the Thermal Energy turns to Potential Energy.
The horizontal portions of the graph represents Phase change and the temperature's constant state