Marx said that it is not consciousness that determines life but it is life that determines consciousness. Think of it that way: Our physical functions and interactions will determine the ways we think about the world. More specifically, the Base consists of these physical functions and interactions, or what Marx would call the material conditions of existence. These material conditions, the Base, are socioeconomic; our monetary system, economic system, how and why we buy and sell property (everything from food to housing to clothes – the means of survival, and later luxuries). This base gives rise to the Superstructure which is our philosophy, religion, art, law, politics. The Superstructure, once created, can reciprocally influence the Base, but the Base largely controls the manifestations of the Superstructure. So, the Base is life (our actual material conditions of existence, namely socioeconomics). And the Superstructure is our collective consciousness.
Marx claimed that the Capitalist system’s Base creates a Superstructure in which the ideology of the bourgeois (ruling class) dominates cultural spheres and this domination is called hegemony. Members of the working class (Proletariat) who unknowingly buy into these ideologies unknowingly participate in the perpetuation of their own oppression and thus, they exhibit a “false consciousness.” The working class is oppressed because their surplus labor/surplus value all becomes profit which all goes to the ruling class (bourgeois). These material relations (Base) of production come into conflict with the social forces of production (Base) and this could lead to a social revolution because this would be a change in the Base – change to a conflict. Remember that the Base gives rise to the Superstructure, life gives rise to consciousness. So, if a social revolution began, it could be a gradual transformation of that socioeconomic base but it could also be a quicker, more forceful revolution (Cuba) as the Base creates an element of Superstructural consciousness that brings the Proletariat out of its “false consciousness.” This would be a case where the Base and the Superstructure change together. And in this case, the consciousness of the Proletariat could proactively work to change that Base.
Remember that the Base is actually a simple concept: it is just the stuff we do every day, the literal, physical stuff. This largely determines the Superstructure which is analogous to consciousness. More recent interpretations of this Base-Superstructure model have shown that the model is more reciprocal in its function depending on the period and place in history.
To sum up, Marx says that the way we think about the world (Superstructure) cannot change until the world itself changes (Base). Marx said that the contradictions that would arise in late Capitalism would actually provide the means for such a revolution. In other words, the unequal distribution of surplus value in Capitalism (which is what Capitalism is based on: profit) would eventually lead to these conflicting changes in its base and would result in a transformation to Communism.