Explanation
A statement about what is there, what it does, and how and why. Predictions about what is going to happen next are insufficient explanations.
Good Explanation
A good explanation is Hard to vary while still accounting for what it purports to account for.
Hard to vary
A hard-to-vary explanation provides specific details that fit together so tightly that it is impossible to change any detail without affecting the whole theory. A Good Explanation is hard-to-vary whereas a with a Bad Explanation you could vary the details without affecting the explanation.