Though known for scanning submissions for plagiarism, Turnitin actually does not check for plagiarism in your work. What it does is check your submissions against its database, and if there are instances where a writing is similar to, or matches against, one of its sources, it will flag this for you as instructor to review. Turnitin's database includes billions of web pages: both current and archived content from the internet, a repository of works other students have submitted to Turnitin in the past, and a collection of documents, which comprises thousands of periodicals, journals, and publications.
The following links should be helpful in understanding and interpreting Turnitin's Similarity Report:
For more help with Turnitin, please visit TurnItIn Instructor Manual for Canvas