why is heat energy needed to melt a solid what is this heat energy called

Heat energy is needed to convert solid into a liquid because heat energy increases the kinetic energy of the particles. As the kinetic energy increases, the movement of the particles increases and eventually the bond or attraction between the particles gets broken and the particles start moving faster. Moving faster cause the state of the matter to change becasue the particles in solid are rigid but when heat is applied the particles are loosened and start moving freely. So it chenge into a liquid state. The heat energy that it used to change 1 kg of solid into liquid at atmospheric pressure and at its melting point is called the latent heat of fusion. In other words it is heat that is needed to melt 1kg of solid into liquid.

I hope it helps.