The rating system uses a 5-tier system:
Very High – The best there is. Considered a must watch.
High – Easy recommendations unless the genre really isn’t for you.
Medium – Average, decent work. A solid pick, but with clear flaws.
Low – Difficult to recommend unless a diehard fan of the genre.
Very Low – The worst there is. Some may be so bad they’re good.
These ratings also apply to sub-categories Art, Sound, and Story for those interested in particular aspects. These sub-category ratings don’t average out to the Overall rating – Overall is independent.
Remember that the rating is just a summary. I recommend that you always read the review for the full picture; a lower tier may be due to a feature that doesn’t bother you.
(You can read about the review methodology here.)