5 - Decent
This anime is neither good nor bad. People can watch it whenever they are bored or when they've run out of other anime to watch.
4 - Bad
This anime is just plain bad. It has nothing to recommend, it is just plain boring. While there are worse animes it's still too bad to recommend to others.
3 - Very Bad
This is a very bad anime although still anime. There's nothing appealing about it. You cannot recommend it to anyone, not even die hard anime fans of this specific genre. If you think an anime is one of the worst ever vote a 3.
2 - Hopeless
You cannot say anything good at all about this one. Animation, sound, story, everything was really bad.
This grade is reserved only for the very bad cartoons. There is probably nothing as bad as this in AniDB. Use this only if you're absolutely certain.