The document discusses the evolution of the web from a network of linked documents to a network of linked data. It describes how early efforts at creating linked data involved projects like the Library of Congress linking photograph collections. It then discusses how the concept of linked data grew out of semantic web research in the 2000s and was popularized by Tim Berners-Lee in 2009 as a way to make data on the web more open and useful by linking related data items. The document emphasizes that linked data is about using semantic web technologies like RDF to not just publish data but to describe relationships between real-world things.