'Marxism' was a theory created by someone called Karl Marx about society. The theory suggests that western societies, like Britain, are controlled by the economy. This means that all our institutions, like the education system, politics and law are all influenced by the economy. Marx says that because of this, society is unequal.
Marx explains this further by saying there are two types of people in society, the 'bourgeoisie' and the 'proletariat'. The Bourgeoisie are the rich people, they have control over the economy, by owning businesses and being managers and people who have high status jobs (like politicians). The Proletariat, according to Marx, are those who do not have access to the economy (or what Marx calls the means of production), they are made up of normal everyday workers. Through giving these two categories of people, Marx explains how society is unequal as the proletariat are unable to become the bourgeoisie. This is because they do not have the knowledge or social status to get to these jobs, so they are trapped in low paid jobs - Marx calls them 'wage slaves'. Overall, Marxism is an argument given to explain why society is the way it is. Many other people follow this school of thought, like Pierre Bourdieu and further enhance this argument.