In Stave I, after having his "melancholy dinner," reading the newspapers, and spending time with his banker's book, Ebenezer Scrooge returns to his quarters in a "gloomy suite or rooms." As he places his key in the lock, the knocker transforms and appears to possess the face of his dead partner, Marley. The face is not in shadow as everything else, but it "glowed like a bad lobster in a dark cellar"; that is, the face is livid, pale and sickening-looking, as a spoiled lobster would be a greenish-gray color. The eyes of lobsters are blank and sit atop the head of the crustacean; Marley's glasses rest on his forehead in much the same way and his eyes, too, are immobile, and his face is as motionless as if he has a shell like the crutacean. Thus, the horror of Marley's face lies in its livid color and crusty motionlessness--its resemblance to the rotten lobster cast away into a cellar.
When the story opens, we learn what a miserable man Scrooge is, and we also learn his business partner, Marley, has died and Scrooge has taken over the entire business. After a long day at work and having meetings and a very dismal dinner, Scrooge is heading home. As he puts the key in the door, the door knocker's face turns into the face of Marley. Scrooge explains it as looking like a lobster in a dark cellar. That means that it looked pale and lifeless, like a lobster would look like after death.
Marley is appearing to Scrooge to try to save him from the same fate that he has received. Marley is trying to warn Scrooge of the path his life is taking him. Scrooge tries to ignore him. He explains him away as being something he must have eaten. Marley warns him that three spirits will visit him to show him the way he is headed. When Marley was alive, he and Scrooge were very much alike. They only cared about how much money they could make, and it didn't matter who they hurt in the process. After Marley dies, Scrooge continues living the way he always had, Marley has come back to try to save Scrooge.
Dickens had such a great way with words. The way he describes certain things is magical. Comparing the face of the long dead Marley to a lobster in a cellar, is just brilliant. Dickens really tries to make us realize that life is much more than how much money one person may have, he wants us the realize the importance of treating other people with the respect and dignity they deserve. This is what the three ghosts show Scrooge and thankfully he sees the error of his ways and makes things right.