The key into the other world becomes a hot commodity, or item, in the story. There's only one key to the door between the other world and our world. And the other mother and Coraline both want to control that key.
The appearance of the key can help us understand what it represents. Compared to the other keys, it's "the oldest, biggest, blackest, rustiest key" (1.51). It's different from all the other keys, just like the other world is different from the real world. In this way, the key seems to represent the other world: old and ugly.
Usually when we think of keys, we think of unlocking something or opening something up. Coraline certainly does this when she finds the other world. But if keys can unlock, they can also lock you in. Eek! That's more what Coraline is about – the fear of being trapped somewhere you don't want to be. So how do we eliminate the fear of being trapped? Well, trap the key itself! And that's exactly what Coraline does.