A timeless reminder of the evils of big organisations and the gullibility of human beings.
The inspiration for Francis Ford Coppola's 1979 Vietnam war film, Apocalypse Now, this was originally written in 1899 as a commentary on European colonialism and is based on Conrad's own experiences as a steamboat captain on the Congo.
The book's message is the inhumanity of colonialism and how apparently benign structures, in this case a European trading company, can support and tolerate disgusting and ugly practices.
The plot has Marlow, a steamboat captain, take a job on the Congo with a Belgian trading company. He travels up river to supply Kurtz, who is said to be a remarkable man and controls the upper part of the river for the company. His methods of control turn out to be extreme violence, but Marlow is more impressed with Kurtz's intellectual honesty about the situation than the company's pretended good governance (for example the ship's native crew are paid in copper wire with which they are supposed to buy their own food but which in practice is untradeable and so they starve to death). Marlow falls in love with Kurtz in the way that supporters fell for (say) Hitler.
It's pretty strong stuff and one assumes it is meant ironically to point out the ultimate logic of the colonial position.
It's not an easy read, partly because it's not always very well written (unlike, say, Conrad's The Secret Agent) but mainly because the themes and events set out are approached obliquely. Quite what is going on between Kurtz and the Company and between Kurtz and the natives is never spelled out. Nor is it clear why Kutz is widely regarded as a great man, who has attracted a band of devoted followers. This doesn't matter to Conrad's themes, but it's never obvious where the book is going and some of the scenes and interactions are open to any number of interpretations. This is probably the book's attraction and fans of ambiguity will enjoy mulling over the possible meanings. Others will find it frustrating.