Stratification is the phenomenon in which the people in a society are differentiated in terms of the amount of wealth (or other resources) that they possess. In other words, stratification exists if various groups in a society have very different levels of wealth, power, or other important resources.
Issues of stratification are quite central to much of sociology and to other social sciences as well. Sociologists might ask, for example, why African Americans tend to have less wealth, less education, etc. than white Americans do. Social scientists might also ask why women are less represented in positions of power than men are. Both of these are forms of stratification.
So, as the link below says,
The term stratification in sociology is usually applied to studies of structured social inequality; that is, studies of any systematic inequalities between groups of people, which arise as the unintended consequence of social processes and relationships.