How is the idea of nature relevant to the novel Frankenstein?
By "idea," I assume you mean "theme." The theme of nature in Frankenstein is one of the most significant in the novel and is presented in many ways. Sometimes Victor associates himself with nature or a force of nature. Sometimes he is appreciating nature itself. Many times he is trying to control nature. Many have argued that the book's message is about how humans should not disturb the forces of nature.
Here are a three specific examples (there are SO many more, though) of nature being presented in the novel:
- In Ch. IV Victor notes that he is "engaged heart and soul in one pursuit," and as a result he misses out on "a most beautiful season" of summer. He describes the season and his surroundings, but notes that he is not able to partake in any of it due to his studies.
- In Ch. IV Victor actually describes himself as a force of nature: "No one can conceive the variety of feelings which bore my onwards, like a hurricane, in the first enthusiasm of success. Life and death appeared to me ideal bounds, which I should first break through, and pour a torrent of light into our dark world." In this quote we see he is both hurricane and sunlight. He is both a storm of energy and a ray of light.
- In Ch. VI we see Victor and Henry Clerical get back to nature, so to speak, by taking a walking tour of Ingolstadt. Here, Victor seems to truly reconnect with nature and remember all of its beauty that he has been oblivious to why he has been studying.