Historians in Marx's time thought that any human civilization could be explain by the products of the Spirit/the mind; this means arts, religion, politics, philosophy, humanities, etc. Marx said that, since men are work, the only thing that explains any civilization is the material or living conditions of working men. Society consists of the base/infrastructure (this is, the work) and an ideological superstructure (the intellectual stuff).
The base consists of two parts: productive forces and relations of production. Productive forces mean workers/proletariat, technologies, sciences, tools; everything used for working. Relations appear when you have somebody owning the tools or technologies. These are the relationships built around production (for instance, owner and worker). Marx stated that productive forces evolve faster than the relations.
An example of this would be the music industry nowadays, and their position on downloading songs for free. We have the technology to be able to produce our own songs and distribute them online for almost no price, and yet record companies want to charge you for downloading, instead of thinking of other ways of making income (like concerts, merchandizing, I don't know). The thing is, relations of production evolve slowly because, if they change, it means people with power will lose it.
When the productive forces have evolved so much the relations become a burden, something called "revolution" occurs, which makes the relations change and History moves forward. This is what Marx called Historical Materialism, since it depends on the material conditions of the workers.