The book explains that apart from the subjective and objective realities there is intersubjectivity. This third reality is often forgotten or rather it's not known.
We usually make the mistake that if a concept isn't subjective we conclude that it must be objective. Taking the example of money which is not subjective but is neither objective, it is intersubjective. We as a society have made money valuable by thinking that it has value and building social structures around the concept of money.
Intersubjectivity thus is a reality which most people adhere to and so are accepted as a societal reality.