The attraction between the particles are very strong and need a lot of energy to break the bonds.
Ionic compounds have very strong attraction between particles, thus a lot of energy is required to separate them. The opposite can be said about boiling point.
The attraction strength is a result of the positive charge of the cation (metal) and the negative charge of the anion (non-metal) transferring their electrons to achieve a full valence shell. This process ultimately results in a strong attraction of particles.
Elements alone, metals tend to have high melting points while gases tend to have high boiling points.
Energy = heat energy in this case.
Hope this helps :)