The "secure" window tries to provide a little more security for quizzes (making copying and cheating more difficult) by restricting some of the things that students can do with their browsers.
NOTE: This security is not watertight and should not be wholly relied upon. It is impossible to implement complete protection of quizzes in a web environment. Other strategies include the creation of large databases of questions from which you randomly choose questions or the inclusion in the final grade of forum discussions, glossary building, wiki writing, workshops, and assignments.