A piece of music is good if it gives you pleasure. The more pleasure it gives you the better the album. An album is better than another if the pleasure it gives you has higher intensity and longer duration. The grades are attempts to quantify those qualities. Needless to say, the grades reflect the highly subjective view of the reviewer and nothing else.
This is a masterpiece. All the songs are great, and anybody with the slightest interest in music should own it.
This is (or should be!) a classic album. If you are remotely interested in its genre you should own it.
This is a good album. That means good, not average. Every once in a while you listen to it and like what you hear.
This is a decent album. You don't mind listening to it, but it doesn't do that much for you. Or you actually like parts of it quite a bit, but you don't care for the bulk of it.
This grade is reserved for albums that cause the opposite of pleasure. If you sit in a comfortable armchair and your stereo (somehow) starts to play this album you have to get up and turn it off. You simply prefer the silence.