Marxism is a philosophy of history and culture. In looking at history, Marxists focus on the social, cultural, and economic intuitions of a given society and then they look at the ideologies that form and in turn are reinforced by those institutions. Therefore, Marxist theorists look at literature as a social institution as well: part of a society's culture. They investigate the ways a piece of literature emerges from current ideologies and institutions.
Since Marxism is a philosophy of history and historical progress, Marxist theorists often look at the author's background and the author's social and economic world he/she was writing in. Karl Marx said/wrote that consciousness does not determine life, life determines consciousness. In other words, a piece of literature is not something the author solely created him/herself. Rather, an individual (or a writer) is determined by his or her social world. That being said, politics and economics play a large role in the social world and therefore they play a large role in how the individual functions in that society. In investigating the meaning of a work of literature, the Marxist theorist would have to look at how the author was influenced as well as how the content of the text was influenced by ideologies and/or social institutions.
Some Marxist theorists focus on class struggle and this has much to do with politics and economics. If a Marxist focuses on the individual (author or character in a novel), that theorist will have to consider the ways the individual is determined by class, politics, and economics to be a certain way: to play a certain role in society and/or to think in certain ways.
As the focus of Marxist theorists is primarily history and the modes of production (capitalism, economics, politics, class struggle), a rigorous theorist of Marxism should consider how the individual and how the whole society is determined and/or indoctrinated by the social institutions they live in/under. Therefore, Marxism is a theory of history but in investigating the era and historical development of a particular author or a particular text, the Marxist theorist must address how social institutions condition people to live and behave in certain ways; and perhaps even consider how a potential rebellion against those historical determinants might be possible.