According to Karl Marx, the social relations that matter most in a capitalist system are those between the capitalists and the workers. This is because it is these social relations that determine how much profit is generated by the capitalist system.
Marx argued that a capitalist system is one in which there are two classes: the capitalists, who own property and means of production, and the workers, who sell their labor power to the capitalists for wages. The relationship between these two classes is exploitative, as Marx believed that capitalists were able to profit off workers’ labor because they controlled access to the means of production. He saw this as an inherent part of capitalist systems, which he believed were bound to collapse due to internal conflicts between classes.
The capitalist owns capital goods (she owns machines and other materials needed for production), hires labor power from workers, and then sells the commodities produced to make a profit. The value of these commodities is determined by their use-value (how useful they are for satisfying human needs) and exchange-value (how much money can be made selling them).
In order to make a profit in selling commodities, capitalists must pay workers less than they receive from selling the commodities produced by those workers. So there’s a difference between what workers get paid in wages and what they produce in value during their work time. This difference represents surplus value or unpaid labor that goes directly into the pockets of capitalists as profits.