Students' Guide to Turnitin

Match:  The paper's text that is very similar or the same as some text found in Turnitin's database.

• Matches are highlighted in Turnitin.

Source - Where a particular match has been found. 

• Examples of a Source: Website, Student Paper, Published Articles 
• Sources are listed numerically in the insight panel.
• It is possible to have multiple matches for each source.
• Selecting one of the sources will reveal information like how many matches were found for that particular source and where each match was found on that source.

Source: https://help.turnitin.com/feedback-studio/turnitin-website/instructor/the-similarity-report/interpreting-the-similarity-report.htm