When a paper is submitted via Turnitin, an overall percentage of similarity or originality score (with colour code) is shown (see picture below). The similarity score is just a percentage of text in the student’s paper that matches sources in the Turnitin database.  Turnitin highlights the text in the student’s paper that is similar to or matches against another source. The picture below shows a Turnitin Originality Report in the Document Viewer window. On the right of the image, you will see a numbered list of matched sources. Each of these is allocated a colour and number and these correspond to the number and colour in the student’s text. 

Highlighted text may indicate plagiarism and it will be checked by the module teacher. Take note that a high percentage does not mean there is definite plagiarism. There may be cases where the high percentage is due to appendices which are necessary for submission for example the financial statement of a company which is the case study of the assignment.

High similarity percentage may occur if a student copies too much text (exact words and sentences) from another source(s) without paraphrasing. This is called direct copying or verbatim type of plagiarism. See the red highlighted text numbered 1 in the picture. High similarity percentage may also occur if there is poor paraphrasing of text from another source(s) by student – see the pink highlighted text numbered 2. Both these high percentages are grounds for further investigation by the teacher on the student’s paper.

Likewise, a low similarity percentage does not mean there is no plagiarism. It is best that you check your paper thoroughly after submission to ensure that there are not many highlighted text. 

Source: www.wittenborg-online.com