This transforms the solid into liquid state. As more energy is given to the substance, the bonds stretch even more and ultimately break. The liquid then is turned into gaseous matter. The inter-molecular forces become weaker as we go from solid to liquid to gaseous state. Heat has to be added or taken away for a substance to change its state.
L denotes latent heat.
The latent heat for solid to liquid is known as the latent heat of fusion. The latent heat from liquid to gas is known as latent heat of vaporization. When you press a small piece of ice in between your fingers, the ice starts melting. The energy is quite high and you will feel as if your fingers are burning. Thus the burning is due to energy locked up in the ice, which is given off on melting.