When an object is changing forms (solid to liquid in this case), the temperature remains constant. All of the energy that would normally go towards changing the internal temperature of the object is going into the latent heat of fusion or enthalpy of fusion instead.

Enthalpy, or , is the total energy of a thermodynamic system. Similar to how mechanical energy can change during mechanical processes, involving changing distances of velocities, enthalpy will increase or decrease with changes made to the thermodynamic state of the system. It is simply a measure for a different form of energy.