Imperialism, or the aggressive colonization of one or more countries by another in order to exert control over their natural resources, is the political context against which Heart of Darkness is set. Specifically, the book was inspired by Conrad's experience as a riverboat captain in the Belgian Congo, which was being ruthlessly exploited by Leopold II for ivory and rubber.
The unnamed "company" Marlow works for is an example of this sort of imperialism. There are many examples, but the main idea common to them is that white people from Europe impose a government on an arbitrarily defined area, declare it a "colony," then exploit (or, in the case of the Congo, essentially enslave) the local population for cheap labor.
Conrad's story includes many details showing what imperialism looks like up close. The famous "grove of death" scene, in which Marlow finds a patch of shade where exhausted African laborers go to wait for death, is one such example. The station accountant, whose insistence on wearing European clothes in the African heat, is another. Time and again, European imperialism is seen as absurd and horrifying. Kurtz's outpost upriver itself is a kind of viral imperialism; he has set himself up as a kind of independent authority that nevertheless is as exploitative as the company.