A progress quiz (previously referred to as a certificate test) is a language proficiency test, centered on the material covered by Duolingo's courses, available for purchase with lingots in the Lingot Store.
The test covers all of the content in the course, not just what has been learned so far. There is a lingot cost each time a quiz is taken but no other limit on how many times or how often it can be taken. Hover hints are not available in progress quizzes.
After you've completed the quiz, you will be shown your score, which is given out of 5 points. You can also view your scores from previous quizzes by hovering over your username on the blue bar at the top of the page and clicking "Quiz History".
The quiz serves as a quick and informal test of your proficiency in the course material that Duolingo teaches. For a test of more advanced material, as well as one that results in a verified certificate that can be shown to prospective schools or employers, you may want to consider Duolingo Test Center.
Some users find taking progress quizzes periodically is a useful way to track their progress in learning a language.