The Checkit Service uses Turnitin software to compare researcher-submitted papers to billions of internet documents, including archived data that is no longer available on the live web and an increasingly comprehensive database of previously submitted papers and then produces an 'originality' report.
There is a lot of confusion surrounding these reports. It is worth noting that the headline score reported is a 'similarity score' rather than a plagiarism score. In other words, it indicates the percentage match between your work and that found by Turnitin. For example, a 20% match doesn’t mean that 20% of your thesis is copied. It simply indicates that 20% of the work you have uploaded is matched to other sources in Turnitin's database.
Understanding what that 20% is made of and why it matches is much more important. Does it contain a 15% match to a single source from another paper that isn’t referenced, or is it lots of smaller matches to content that is properly referenced or uses set terminology/phrasing? This is why it is important that your supervisor closely reviews the report, looking at the context to try to ascertain the intent of the author.