Karl Popper's book is an outstanding critique to what has become known as historicism, the belief that the future of society can be accurately predicted, and that history follows a fixed path towards a specific goal.
This idea is mostly associated with two influential figures. The first is the German philosopher Hegel who produced a philosophy of history based on the tradition of philosophical idealism, and the other is Karl Marx, who, strongly influenced by Hegel, created the concept of the "materialistic conception of history", or as is commonly known today, dialectical materialism. In this concept, Marx laid out the theory that history is pre-determined to move towards its ending point, which is a classless, stateless, propertyless society he described as "communism". Marx believed that he had uncovered the scientific laws of history and thus could predict its development and its end. The main problem arises when such ideas escape the scientific community and are translated into political action. Hence Communism was driven by Marxist historicism, and other oppressive regimes, such as the Nazis, had a touch of racialist historicism in their ideology.
One needs to understand that not all of what Marx and Hegel wrote was plain historicism, and their methods and theories have been adopted in various disciplines, based of course on their relevance. But this notion of a fixed historical road, or the idea that one can clearly predict the major events of the future, is what Karl Popper objects to. In this book, he explains with clear logic, that such ideas are unscientific, and should be included in the trash-can of pseudo science. Popper's argument is strong and to the point, demolishing the scientific and the logical basis that historicist theories claim to have. I strongly recommend this book, for it demolishes a branch of thought that has stayed way too long with us, and has caused, sometimes intentionally, but many times unintentionally, great harm to humanity.