lots of really good answers in this thread, many of them more academic and thorough than mine, but my relatively simplistic version is that the imperial core is the collection of nations with the means and will to project force on all of the rest of the world (i.e. those nations who have the means to execute imperialism) and therefore the nations who ultimately reap the rewards of holding power over subjugated nations. That can be raw materials like lithium and oil, crops, cheap or slave labor, asymmetric rights to land and passage, or any of a number of other explicit and implicit hierarchies that the imperial core is at the top of.

This is very similar in concept to the first/third world, the developed/developing world, and the global north/global south, although they all have slight but important differences in meaning and history.