Marx employed a labour theory of value, which holds that the value of a commodity is the socially necessary labour time invested in it. Capitalists, however, do not pay workers the full value of the commodities they produce. The gap between the value a worker produces and her wage is a form of unpaid labour, known as surplus value. Moreover, Marx notes that markets tend to obscure the social relationships and processes of production, a phenomenon he termed commodity fetishism. People are highly aware of commodities, and usually don't think about the relationships and labour they represent.
Marx's theory of value, perhaps his most important contribution to the field of economics, albeit, the most rejected - stated that the value of any given commodity is determined by the socially average simple labour time used to create it, giving skilled labour value in multiple units of unskilled labour, suggesting that the market determines all prices based on this mythical underlying labour cost