Relations of production
Some social relations are voluntary and freely chosen (a person chooses to associate with another person or a group). But other social relations are involuntary, i.e. people can be socially related, whether they like that or not, because they are part of a family, a group, an organization, a community, a nation etc.
By "relations of production", Marx and Engels meant the sum total of social relationships that people must enter into in order to survive, to produce, and to reproduce their means of life. As people must enter into these social relationships, i.e. because participation in them is not voluntary, the totality of these relationships constitute a relatively stable and permanent structure, the "economic structure" or mode of production.
Throughout history social property appeared in the form of the property of the clan and the tribe (primitive communism), public or state property (socialism), cooperative and community property (communism), etc. – while private ownership appears in history through: slave property, feudal property, and capitalist property. From all forms of ownership correspond lesser or greater types of exploitation of human by human.