User overrides allow you to set an exception to the rules for a quiz or assignment for a specific user. For example if you have a student that needs more time on a timed exam, a user override will allow you to add more time for that student (without changing the quiz settings for everyone). It can also provide deadline extensions, etc. Read more about it at http://docs.moodle.org/26/en/Quiz_settings#Group_and_User_overrides