It deals with a homogeneous mixture of (l) + (s)
The image shows a method to evaporate carefully (on boiling water in a water bath). Such a water bath can only serve as such if a temperature of 100ºC is enough to evaporate the liquid. The dissolved substance will remain behind.
When the particles of a liquid receive more energy, or even when lattice particles receive energy, (for example because the substance is heated), than you increase the movement and the vibrations of those particles.
At a certain moment this movement will become so heavily, that the particles cannot longer stay toghether: The internal attraction forces are no longer sufficient to keep them toghether.
When they become completely loose from each other, so a gas is formed, those particles still can collide. But even so, they will not stay toghether after the meeting because of the energy.
Of course, they will stay toghether again at cooling down. Then the substances can become liquid or solid again.