One of the defining features of Marxism is its methodology, or how it goes about analyzing the world. The methodology Marxism employs is Historical Materialism. At its most simple, Historical Materialism is an attempt to look at the development of societies through an investigation of material conditions. Material conditions is a term that refers to the ways that a society produces goods, as well as the technological development that makes this production possible. By looking at material conditions, Marxism is able to analyze the ways that societies attempt to provide food and other commodities for their members.
As an example of this, Marxism looks at the development of capitalism as a shift in material conditions. Marx used Historical Materialism to examine industrialization as a massive shift in productive capabilities and technologies, controlled by a capitalist class. From this perspective, capitalist society developed around these conditions.
Marxism holds that Historical Materialism is a tool that can be applied to understand pre-capitalist society as well. Marxists look to the organization of production through a serf and peasant class, as the defining material condition of Feudalism, for example.
In short: Historical Materialism is a tool of analysis which looks to material conditions as the basis of a society, and assumes societies develop from those material conditions.
It is important to recognize that Marx’ Historical Materialism is, well, just that: materialism. It developed in response to the Hegelian idealist tradition. In Hegel’s philosophy of history, culture and human society develop as realizations of what he terms the Spirit. We won’t get into the nuances of Hegelian idealism here, but it’s worth bringing up as an example of an idealist philosophy of history: rather than being produced and changed by the material conditions, in Hegel, history is advanced by changes in the Spirit.
In Marx, the communist project is not an ideal to be achieved; Marx’ work did not attempt to outline an alternative, moral or ideal society towards which the workers must strive. In Marxist thought, communism is not the product of some grand realization of the Spirit. The Materialist method instead locates communism as something which emerges from the real material conditions of capitalism.
In other words, the Materialist method of social critique looks at the tensions of capitalist conditions (the wage-form, alienation, and others) and traces the birth of revolutionary fervor from those tensions. It seeks to locate a material basis for radicalism, rather than a moral or ideal basis.