A - Top notch and stellar. This game accomplished everything that it set out to do and then some.
B - Great! A game worthy of a B means it is a requirement for any gamer to get. Only slight set backs seem to hit a game like this.
C - Games that get a C rating are good. They may not be great. They may not be fantastic. They are fun, interesting, and worthy of a play-through.
D - Average. Sometimes games play it too safe, sometimes they don't step out of their bubble, and sometimes they have too many problems. But they are playable and you may still find fun. Those games deserve a D.
F - Don't play this. The game failed in what it was trying to do. It's not fun and it's not compelling. The game may or may not be playable, regardless, if it has an F, it's probably safe to say you shouldn't play it.