Turnitin is a tool which highlights similarity.
It does not determine if a paper has been plagiarised - that is the prerogative of the assessor.
Ordinarily, Turnitin will highlight any similarity to other sources.
The assessor (instructor/supervisor) has to decide if the similar text has been quoted correctly and attributed accurately.
The context is important, and Turnitin does not account for context.
To reiterate, Turnitin merely detects similarity to other sources.
A high Similarity Index could mean that the student has legitimately quoted many other works to a significant extent.
Also, common phrases in a particular field may be highlighted as similar to other sources.
Another common reason is that Turnitin is comparing a student's submission to his/her previous submission.
Whether or not this makes a good essay, just like the question of whether a student has plagiarised a paper, is a decision that the assessor has to make.
Turnitin does not determine whether a paper has or has not been plagiarized. Similarity Reports are simply tools to help instructors locate potential sources of plagiarism, or text which may have been incorrectly cited. Therefore, only instructors can deem what is a "good" or "bad" score, as interpretation of the data can only be made by the instructor.
http://turnitin.force.com/pkb/articles/en_US/Knowledge_Article/Similarity-Report-Interpreting-Results-Instructor/ (retrieved on 13 Apr 2016)
[The Similarity Indices] do not reflect Turnitin’s assessment of whether a paper has or has not been plagiarized. [Similarity Reports] are simply a tool to help an instructor find sources that contain text similar to submitted papers. The decision to deem any work plagiarized must be made carefully, and only after in depth examination of both the submitted paper and suspect sources in accordance with the standards of the class and institution where the paper was submitted.
http://turnitin.com/en_us/training/instructor-training/about-originalitycheck (retrieved on 18 Sep 2014)