Ratings are on a scale of 1 to 5, on something of a bell curve, so that 1's and 5's are rare and 3's are most common. As for what they mean:
- Poor. Bad. Incompetence in writing, art, or both. Not of interest to anyone other than the creators' relatives.
- Okay. If you're a big fan of the genre and/or the creators you may find this of interest. Otherwise you should pass.
- Good. Solid entertainment. If you're a fan of the genre and/or creators involved you will probably like this.
- Great. Must reading for fans of the genre and/or creators, and probably worthwhile for others as well.
- Excellent. The crème de la crème of comics art. Should be read by anyone with an interest in comics, and probably by those who think they have no interest as well.
So there you go. As can be seen, the ratings are somewhat relative. A good solid super-hero book will get a 3, but if you're not interested in super-heroes you should probably give it a pass. If a volume of manga gets a 4, you may want to consider picking it up, even if you're not normally into manga. And so it goes.