For a phase transition from a liquid to gas or from a solid to a liquid, it is necessary to apply heat etc. externally. In fact, during this phase change, the temperature of the substance is constant and does not change. For example, water stirred with ice is kept at a constant temperature of almost 0°C, while water boiling at atmospheric pressure is kept at about 100°C. Thus, during the phase transition, the temperature of a substance is not constant and does not change.
Thus, during the phase transition, the added thermal energy is transformed into a structural change in the material, which is called latent heat because it does not involve any apparent thermal change, and thus is called "latent heat" from the meaning of "submerged and invisible" thermal change.
Latent heat of evaporation refers to the heat required for the transformation of a liquid substance into a gaseous state.