Realists believe that many entities exist independently of us and our investigations of them. The realist social scientist claims that social entities (like markets or social rules) exist independently of our investigations of them.
Realists believe that systems are complex that exists independently of our knowledge of them. A practical example, an organisational structure will exists even without explicit design of it. It is a product of human beings, even if it not is designed.
Contrary to Postmodernism, realists acknowledge the social constructed nature of the world, but don't see the world as being merely a social construction. Systems are a causal mechanism which has the potential and capability to act in a certain way.
For example, a person might have a friendly relationship with a bank manager. That will not mean that person will get a loan. The bank manager still has to follow the procedures and check the creditworthiness of the person. Nevertheless, the bank manager has the ability to exert judgement and a degree of discretion in what they do.
Bhaskar argues that nothing happens out of nothing:
People do not create society. It pre-exists them and is a necessary condition for their activity. Society must be regarded as an ensemble of structures, practices and conventions which individuals reproduce and/or transform, but which would not exist unless they did so. Society does not exist independently of human activity. But it is not the product of it.