Using the power of Search, originality reports compare student work against hundreds of billions of web pages and tens of millions of books. The reports highlight missing citations, ineffective paraphrasing, or unintended plagiarism due to high similarity and link to the external source.
Students can run originality reports up to three times before submitting, allowing them to identify and address citation errors, and saving you time on grading. You get your own report with each submission so you can provide additional guidance if needed. (From Google.Support)