The Similarity Report explained
A typical submission made to an assignment in Turnitin generates a Similarity Report. The Similarity Report is the result of comparison between the text of the submission against the search targets selected for the assignment; this may include billions of pages of active and archived internet information, a repository of works previously submitted to Turnitin, and a repository of tens of thousands of periodicals, journals, and publications. Any matching or highly similar text discovered is detailed in the Similarity Report that is available in the assignment inbox.
The similarity score is a percentage of a paper's content that matches to Turnitin's databases; it is not an assessment of whether the paper includes plagiarized material.