Turnitin Originality Report explained
The Turnitin Originality Report is the result of the comparison between the text in the assignment, the search targets selected for the assignment (e.g. web pages, journals, assignment previously to Turnitin) and a massive content database. Any matching or highly similar text discovered is detailed in the Originality Report.
The similarity index is the percentage posted on the top right of the page. This indicates the proportion of the student's work that has similarities with what's in the Turnitin database or online.
The match overview lists all the sections in the student's work that are similar to other sources, in order of percentage of similarity. A high percentage does not necessarily indicate plagiarism. It simply means that the text in the student's paper is similar to other sources, which sometimes occurs for legitimate reasons.
For further information, see the Library's Turnitin guide.