Goes in hand with the notion that the core of capitalism is exploitation. The output produced by a worker is greater than the actual price of the workers salary. The difference between the value generated and what is paid out is the surplus which is then consummed by the bourgousie or owners /capitalists.

The owner class (exploiters) use the labor of the productive class (exploited) for their own benefit, or the owner class hoards the profits of the production from those who were involved in the actual production process. Whether underpaying (unfair compensation) or not allowing the lower class from accessing the goods (i.e. a farmer has workers sew, grow and harvest apples for them, but doesn't give fair pay or access to the apples)