What is the driving force of history? Who makes history? Is it the presidents, generals, and industrialists that we are taught about in our history classes? Is it the ideas people have?
Historical materialism is a theory that hopes to make sense of human history using a scientific methodology. It's adherents point out that before there can be politicians, generals, industrialists or even grand ideas people first need to acquire the necessities of life. The way we go about organizing ourselves to acquire these necessities is going to be the basis from which all of the those other things are developed.
Historical materialism helps explain why society takes the shape that it does. More importantly it has been a tool utilized by those who have recognized the injustices of our society and have fought against them.