A solid state has closely packed molecules than liquid state. In the process of the melting, heat is absorbed from the surrounding to increase the intermolecular distance between the molecules to change the distance of the molecules.
The amount of heat energy required in the process of melting for a unit mass of solid to be converted into liquid is called Latent heat of fusion.
Heat Energy Q=mL
where m is the mass of the substance and L is the latent heat of Fusion
SI unit of Latent Heat of Fusion is J/kg.