A bad explanation is unspecific. You can change any or all of the details without destroying it. Examples of bad explanations:
A statement about what is there, what it does, and how and why. Predictions about what is going to happen next are insufficient explanations.
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.
Problems are inevitable because the search for Good Explanations will create new problems and our Knowledge will always be incomplete. Some problems are hard, but it is a mistake to confuse hard problems with problems that can't be solved.
Science is the practice of finding good Explanations. Science is a way of dealing with theories regardless of whether or not one believes them. They are judged according to whether or not they’re good explanations. And if a particular explanation ends up being the only explanation that survives the intense criticism that reason and science can apply, then it’s not so much adopted at that point as just not discarded. It has survived for the moment.