The answer is: Wendy reveals her familiar knowledge of characters who live in the land of Peter Pan.
By recounting the existence and characteristics of fairies to Mrs. Darling, Wendy shows that she has knowledge of the magical world of Peter Pan. This contributes to the plot by setting up the belief in magical creatures and the existence of a fantastical world that will play a significant role in the story as it progresses. Wendy's knowledge of the fairies also foreshadows the involvement of other mythical beings in the storyline.