What does the quote mean if man define situations as real they are real in their consequences?

it is the thomas theorem I would like to add: It means that if you believe something is real, for instance Santa Claus, the conseuquences of this believe are real. Children believe Santa Claus is real and they and their surroundings are acting according to this believe. In this example: the child defines Santa Claus as real, and this bring a long serious consequences (people spend lots of money buying presents). Of course it is a simplifying answer. It's about the ''social construction of reality".