Google Knowledge Graph is a system that understands facts about people, places and things and how these are all connected, structuring them in a graph. It is used by Google to enhance search engine's results with information gathered from a variety of sources. In other words, it's Google's own database, getting together all the data collected from billions of wide web searches.
You'll probably have seen the Knowledge Graph Card, which appears on the right-hand of your screen after entering a search term. This card contains relevant, context-specific information regarding your search, providing a direct answer to your query.
If you want to get your business' content in the Knowledge Graph, you have to be relevant on your subject. For that you need to have an excellent content, an optimised site, and suitable keywords. That's when you should use the Knowledge Graph optmisation, which is the process to structure your data and mark up important elements of your page, so that it's easier for Google to understand what is all about. That way, Google can tell exactly which meaning you're using, based on your meta description, title, keywords and content; and then, send more traffic to your website and have more visibility.