We know that matter exists in different states like solid, liquid and gaseous state.  The energy required to change the state of a substance is known as a latent heat as latent means hidden.

When the state change is from solid to liquid we say it as the latent heat of fusion, and when the state change is from liquid to a gas, we must use the latent heat of vaporisation.

This energy required is Q= m L, where m is the mass of the substance and L is the specific latent heat of fusion or vaporisation which measures the heat energy to change 1 kg of a solid into a liquid from one state to another state.

The purpose of latent heat can be understood based on its applications in common life, some of which are (a) steaming food (b) cooling drinks with cold water and ice (c) Melting ice on the road by using salt etc.