The Google Knowledge Graph is a box of information that you can locate among the Search Results in the SERP (from Desktop, you can find it to the right of the results).
The layout is similar to that of advanced search results, but it is an agglomeration of information relevant to the search, which Google locates among all web pages.
Through machine learning and semantic analysis, Google reads structured data from web pages and puts it together to create advanced results in the Knowledge Graph.
It is particularly useful for queries from Mobile, because Google’s Voice Assistants respond to user queries by reading the information in the graph.