During the transition from one physical state to another, heat, the energy absorbed by a body is not used to increase the agitation (measured by the temperature) of its molecules, but in breaking the bonding among them.

It continues increasing the thermal energy of the body, but not its temperature.  So, how much energy is needed to produce this change of phase?

The energy needed to produce a change of phase per unit mass is called latent heat (of fusion or boiling, depending on the case).  In the laboratory it is common to measure it in cal/g, but its SI value is measured in j/kg. We should not forget the equivalence: 1 cal/g = 4180 j/kg

In the following visual we will measure the latent heat of fusion and boiling of water, heating a block of ice with an electric heater with a known power.