The quiz preview option allows you to "write" a quiz before releasing it. While previewing a quiz you work through the steps of answering the questions, submitting the quiz, having the quiz graded (if auto-grading applies to it) to ensure that the automatic grading was set up properly, and finally viewing the reports you have set up.
It is always a good idea to preview a quiz before you release it to ensure the accuracy of the content and grading.
Click the 
Preview icon beside a quiz in the quiz list on the Manage Quizzes page to view the quiz as a user would see it. If you have set up a random quiz, the questions are randomly drawn.
Select the Bypass Restrictions box to bypass any restrictions on the quiz that users are subjected to, including conditional release, dates, attempts allowed, and quiz status.