Latency heat is that heat which is either released or absorbed when a substance changes its physical state at constant temperature eg. from solid to liquid at the melting point or from liquid to gas at boiling point.
The change of phase always occurs with a change of heat. However the temperature does not change because that heat energy is used to overcome cohesive forces between molecules of that substance, no part of heat energy is used to increase the kinetic energy of the molecules. Similarly, heat energy is released and molecules of that substance comes closer but temperature doesn't decrease. So its temperature will not rise/decrease since the kinetic energy of molecules remains the same.
It is a horizontal line along x-axis.
