What Does The Turnitin Similarity Report Mean?
After Turnitin has checked a student's assignment, it will return a Similarity Report which indicates how much was copied from other sources.
The lecturer then needs to decide if the copying is correctly quoted or paraphrased from another source (and so there would be no problem), or if the student has simply copied someone else's work and are claiming that it is their own (which is a big problem and will have consequences).
Of course, we want students to read widely before completing assignments, so they get a better understanding of the topic. But we don't want them to just find something relevant and copy slabs of it then claim they wrote it.
How Much is Plagiarism?
The Turnitin Similarity Report indicates the amount of the student's assignment that has been copied from somewhere else. There is no "safe cutoff" when it comes to percent copied. In the case of a personal reflection, the amount copied should be 0% (unless the student has included the assignment question at the top of their paper).
If the assignment was a research report, then it is expected that quotes and paraphrases from other works should be included. It is up to the (human) tutor to decide whether plagiarism has actually occurred.