Bad artists copy - by that he means they imitate, and repeat popular themes and patterns hoping that they will add value to their work. They don’t really understand why these themes and patterns are so popular or what is their deeper meaning. What they end up is a cheap knock-off work that has very little value.

Good artists steal - they take the idea, theme or pattern and they make it their own. When you steal, you take away the whole thing - this particular idea no longer belongs to the original author. You take it apart, you figure out how it works and you put it together adding your own unique touches. Now it is yours and once you are done with it no one will even remember it used to belong to someone else. You started with something that was not yours, but the end product can no longer be called a copy, imitation or knock of because it stands on it’s own. You successfully stolen something, and gotten away with it.

At least that’s how I understood the quote. It’s the difference between blind copying, and using works of others as inspiration, and/or building blocks of your own work.