A mentor helps a student improve his or her reviews. They can also help to answer any general questions about the site. The way it works is that the student submits a review to the mentor. The mentor then reads over it and sends it back to the student along with comments or questions about what can be added in or left out to improve the review. The student can then edit the review as per the suggestions or add/change anything else in the review. The student sends it back to the mentor to look over again and the process repeats until both mentor and student are happy with the final result. At that point the mentor publishes the review for the public.
To graduate you must do at least two reviews with your mentor. If you need extra work, you can always do more. I have one student that I've done ten or so with (even though her reviews are always awesome she like my input).
Once you graduate you become an Eden advanced reviewer. Then you are eligible for free assignments.
Hope this helps! If you have any other questions, let me know! The mentor program is a wonderful thing.