If you use the Turnitin Assignment tool you will encounter an Originality Report.
For a more in depth look at the Originality Report please see this video resource.
Turnitin uses a text matching algorithm to report a Similarity Index out of 100 which represents the percentage of text that it has matched to other sources. The score itself is not particularly meaningful as whilst plagiarism is more likely to have occurred in a paper that has a high Similarity Score this does not always follow.
Popular topics will generally have higher matches as there will be more material in Turnitin’s database to search against. When reviewing your Originality Report an academic will look at where the matching occurs and how long each match is. For example, if entire sentences or paragraphs match other sources then it is more likely that plagiarism might have occurred versus a report that shows a lot of very short matches. When reviewing a lab report it might be reasonable to expect the introduction, hypothesis and method to be very similar to other papers but we would hope for less matching in the final conclusion.
If you have not deliberately plagiarised then you have nothing to worry about. However, if you do have a high match it is probably worth speaking to your lecturer and revisiting guidance on how to reference correctly.