Google Knowledge Graph is google's own database consisting out of RDF (Resource Description Framework) that structure entities in a scheme which has a graph structure, where all of the data has been collected and evaluated for relevance. It was launched in 2012, and it's a knowledge base created by Google through different sources of information such as Wikipedia, Freebase and CIA World Factbook, to organize information of known people, places and events, and the connections between them. These entities form the nodes of the graph and some of the entities we can find are: Book, Event, LocalBusiness, Movie, MusicAlbum, MusicGroup, Person, Places... All of this information provision would be in meta language.
They are building a massive graph of real-world things and their conections to bring you more meaningful results to understand the search and not just based on keywords, because they know that when you search something, you're not just looking for a webpage, you're looking to get answers, understand or explore. As a result Google Knowledge Graph wants to be able to understand what the user is looking for to give him the best possible information.
In your business you can use the Google Knowledge Graph optimization, which is the process of helping Google to uderstand what your website is about, and you can tell them exactly the meaning you're using, based on your meta description, title, keywords and your content. Once that Google knows what are you writing on your website, they start sending you more traffic and improving your ranking. With Google Knowledge Graph for your Google My Business, they will help you to be more visible in Google, having an increase of visitors.