I’m pretty sure they are referring to imperialism as defined by Lenin, as an evolution and natural consequence of capitalism itself.

I think these paragraphs from ProleWiki: Imperialism give a good introduction:

Imperialism is the most recent evolution of the capitalist mode of production that began in the late 1800s to early 1900s, in which monopolies and cartels become the dominant economic force of society

It is a global system of economic, political, and military domination, with the imperialist powers using a variety of means, including economic sanctions, military interventions, and cultural influence to maintain their dominance over other nations.