I assume that you are talking about the dreams that Victor has at the start of Chapter 5.
Victor dreams here because he has just finished creating the monster. It has just come to life and this really affects Victor.
Victor has all sorts of terrible dreams that have to do with death. He dreams of Elizabeth. He dreams that he sees her walking down the street but when he goes to kiss her, she is dead. He dreams that she turns into his dead mother and he sees the grave worms crawling in her shroud.
I think that this is foreshadowing all the death that will happen because he has created this monster.