Usage of matters being at the phase boundary or even behind the last is a prospective way for a number of modern technologies. Meanwhile, related chapters in the general physic course are usually omitted because of restriction on time. The author presents initial data that are necessary for a student or a novice expert starting to work in the field. Categories of the critical parameters, binodal, spinodal, and metastable region are introduced on the example of van der Waals equation. Probable audience for the paper is general physic teachers preparing their course, or a specialist facing with the questions in the beginning of his scientific or pedagogical activity. The paper can be also used for the self-independent work of students.