Immanuel Wallerstein, a disciple of Karl Marx, developed theories of international relations that constituted scathing critiques of capitalism and rich country domination of less developed nations. Having begun his academic career focusing on colonialism in Africa, Wallerstein was heavily influenced by the experiences of the former colonies both during and after their subjegation by Europe's capitalist states.
As did Marx before him, Wallerstein described a world system in which capitalism is a principal motivator driving wealthy industrialized nations toward an inevitable expansion in search of markets for exports and resources and labor for exploitation. In this respect, according to Wallerstein, the world system evolved into one characterized by stratification, in which the wealthy nations sit atop a socioeconomic ladder, the descending rungs representing less developed and poor nations. Because the wealthy are strong, they are able to enforce this stratified system through force. As Wallerstein wrote in The Modern World System, "Exploitation and the refusal to accept exploitation as either inevitable or just constitute the continuing antinomy of themodern era, joined together in a dialectic [again, the 'Marx' influence] which has far from reached its climax in the twentieth century."
As Wallerstein describes it, the inevitable interaction of countries in itself establishes a "system," and that the dominant characteristic of that system is the ranking of countries according to socioeconomic status. That is what is meant by "stratification."