When a student submits an assignment to Turnitin it generates an Originality Report (aka Similarity Report) a function known as OriginalityCheck in Turnitin. This is a comparison between the text of the student's submitted work against the search of billions of pages of active and archived information previously submitted to Turnitin, and tens of thousands of periodicals, journals, & publications. Any matches or highly similar text is noted in the Originality Report in the assignment inbox. The tutor, you can view the Originality Reports once students start submitting their assessments.
Here is a video that gives an oviewview of the features of an Originality Report.
This video gives more detail on how to use and read an Originality Report.