Hello. You did not inform the book to which this question refers and did not show the passage where the metaphor is presented. This makes it impossible for that question to be answered. However, I will try to help you as best I can.
A metaphor is an implicit comparison, that is, when using the metaphor, the author compares a characteristic between two elements, extending the meaning of one of them and leaving the text more poetic and deep. However, this comparison is not made literally, but must be understood by the reader to be efficient. The strength of a metaphor, therefore, is to allow the reader to understand why the comparison is being made. The weak point, on the other hand, is established when the metaphor fails to make a good connection between the compared elements.