In The German Ideology, Marx asserted that "consciousness was derived from material conditions." Understanding what he meant by this is crucial to understanding his philosophy.
Essentially, Marx argued that to truly understand anything about human societies, one had to grasp the nature of the economic relations that provided their foundation. Everything, including "consciousness," an all-encompassing term that might best be described as a "worldview," was derived from economic relations—what Marx would describe as one's relationship to the means of production. Religion, for example, should be seen in terms of how it upheld existing social class relations, which were in turn based on property ownership. As Marx wrote:
The social structure and the State are continually evolving out of the life-process of definite individuals, but of individuals, not as they may appear in their own or other people’s imagination, but as they really are; i.e. as they operate, produce materially, and hence as they work under definite material limits, presuppositions and conditions independent of their will.
In other words, Marx was concerned with understanding human society in terms of the forces that actually affected their daily lives in tangible ways. These forces were economic.
This approach was a major break from the idealism of German philosophers, heavily influenced by Hegel, who emphasized the primacy of "spirit" and religion in understanding history and society. In The German Ideology, Marx dismissed these as ephemeral, what he would later call "superstructural," reflections of economic, or "material" circumstances. He did, however, accept the Hegelians' conception of history as a series of conflicts between opposing ideas. He just argued that the clashing forces were fundamentally economic classes rather than ideas like "freedom" and "slavery."
Of course, Marx's ideas became very influential in almost every conceivable field. Terry Eagleton, for example, is one of the best known practitioners and historians of Marxist literary criticism, which essentially evaluates literature in terms of the material conditions that produced it (and which, in turn, it reflects). Eagleton argued that Marxist analysis could be applied to literature in several different ways, including anthropological, economic, and political criticism. Whatever the approach, this approach to literary criticism is rooted in Marx's conception of society, and human development, as understood through clashing economic forces.