Why is latent heat hidden?

Hint: Latent heat of energy is the change in energy produced due to a thermodynamic system where the temperature of the system is maintained as a constant. Here the change in energy refers to the absorption or release of energy due to a system to the surroundings.

Complete answer:
We know that a thermodynamic system is where the body of interest is separated from its surrounding by walls. There are different types of thermodynamic systems which exist, each system can be described by the intensive property like temperature or pressure and/or by extensive properties like entropy or internal energy.
In latent heat of energy, the system is kept at constant temperature, and the phase i.e. the physical state of the substance varies or gets converted. The different physical states or phases are the solids, liquids, gases and plasma. The conversion of one phase to another can take place in one big step, i.e. for example solid can get converted to vapour, without passing through the liquid phase, like in latent heat of vaporization.
Clearly, to change the phase of the substance energy is needed; this energy is due the kinetic motion of the particles present in the substance. But since this happens without any change in the temperature, latent heat is also called as the hidden energy.

Note:
Latent heat is the energy used or released by a system when the phase of the system is changed, at constant temperature. A few examples of latent heat is latent heat of fusion, where the solid substance gets converted into liquid. And latent heat of vaporization, where the solid substance gets converted into vapours.