Turnitin works with a database which is said to include:
[…] billions of web pages: both current and archived content from the internet, a repository of works students have submitted to Turnitin in the past, and a collection of documents, which comprises thousands of periodicals, journals, and publications.
(Source: Turnitin Guides)
Therefore it will check your students work against that large database and if there are instances where the writing is similar to, or matches against, one of the database sources, it will flag this for Module Leaders to review.
If you notice 0% next to some of the submissions, it means that Turnitin did not match any of the sources in the database.
It doesn’t mean, however, that the work is 100% original, as the student might have used offline resources (such as printed books or journals) which Turnitin doesn’t match against, nor that the tool doesn’t work properly.
If you are concerned or not sure how to interpret percentages in your submission inbox, please contact Digital Services.