I assume you are referring to Victor's dream after having created the monster and given life to him. Chapter Five details how, having made the monster come to life, Victor is so disgusted and horrified by his creation that he flees to his room, seeking rest in sleep. Instead of finding rest, however, he dreams of Elizabeth, walking in Ingolstadt. Victor is delighted to see her, and embraces her, but as he kisses her, this person changes into a dead corpse: the corpse of his mother. Victor sees a shroud covering this corpse and also the worms at their work underneath it.
Note how this dream foreshadows the death and destruction that the creation of the monster will bring to Victor's nearest and dearest. Having transgressed scientific boundaries, there will be a price to pay, and there is a parallelism in having brought the dead inanimate corpse of the monster to life and the way that the dream strips Elizabeth of life, making her into a corpse that is food for worms.