You can now view the course as a student user would see it. For example, students cannot see the Settings navigation link like instructors can.
You will know if you are in Student View because of the persistent box on the bottom of the screen indicating you are logged into Student View.
You can also reset the Test Student by clicking Reset Student. This will clear all history for the student allowing you to start with a clean slate.
For example, if you created an assignment and submitted it as the Test Student but then forgot to add a certain submission type, you can click the Reset Student button and submit the assignment again as the Test Student.