An explanation of society based on situations where there is a minority persistently exploited by capitalism. Some sort of de facto segregation and exclusion is part of the equation. This was mainly Wales and Ireland. Uses peripheral areas to maintain supplies of cheap labor for capitalist industry. Michael Hechter in the 1970s.
Point is that capital needs to block ability of workers to demand higher wages.
Not like India where there was a great ocean. Capitalists need labor, but need to keep divisions within working class. This is what Internal Colonialism theories are all about. Point is that it is structurally different than mercantile colonialism arrangements where the supply of cheap labor is overseas. Source of cheap labor is next door, i.e. really handy.