Materials come in three phases – solids, liquids and gases. It is obvious that if you heat a sold, liquid or gas, it gets hotter... Or does it? There comes a point where a sold cannot absorb any more heat without melting. While it is melting, to continue melting, heat must still be given to the material. If the supply of heat stops the material will stop melting. While the material is melting, the temperature is constant even though heat energy is being supplied to the material.
While the material is melting, the heat energy supplied does not increase the temperature. Instead it is used to break the chemical bonds in the material that keep all the atoms and molecules in place the the solid. An increase in temperature means the average kinetic energy of the particles of the solid is increasing. The chemical bonds area form of potential energy. Breaking the chemical bonds can mean an increase in total energy without an increase in temperature. When these bonds are broken the atoms and molecules are free to move, and the solid breaks down. It melts and becomes a liquid.
The energy associated with melting is called 'latent heat of fusion'. Similarly, when a liquid is heated to it's boiling point and energy is supplied continuously, the forces between the atoms and molecules in a liquid are overcome. The liquid evaporates and becomes a gas. Energy must be given to the liquid to allow it to evaporate. This energy is called 'latent heat of vaporisation'.
The diagram below illustrates how the temperature of 1 kg of ice changes as it is heated at a constant rate.