A phase transition or phase change refers to transitions between solid, liquid and gaseous states of matter. A phase of a thermodynamic system and the states of matter have uniform physical properties.
During a phase transition certain properties of the medium change, often discontinuously, as a result of the change of some external condition, such as temperature, pressure or others. In this way, a phase transition occurs when energy is supplied or withdrawn from the system in which the material is located.
For example, when we want the water to boil (going from liquid to gaseous state) we supply energy in the form of heat to change its state. On the other hand, when we want to produce ice, we put liquid water in a container that we then put in the freezer that is at a very low temperature, around 0 ºC. By placing the water at a lower temperature the kinetic energy of the water molecules decreases (it is removed from the system) and these molecules are ordered to form a solid (the ice).
Thus, a phase transition occurs when energy is supplied or removed from the molecules.