Say for example, when we heat, water converts to steam gas. How does it happen? What happens underneath giving rise to breaking of bond between molecules in liquid state and spreading them in gas state..?
When you continue to supply thermal energy to a liquid, the mean energy of the molecules (temperature of the system) is increased. Above a certain threshold, the thermal agitation of the molecules are sufficiently large so that a phase change may start.
During the period of the phase change, the two phases (liquid and solid) coexist and temperature does not change. As soon as the whole system has converted to gas the temperature of the gas will continue to increase.
If you want to know more elaborately then ask for it and I will add the details. But it will get technical.