The significance is because the title of it refers to the time elapsed between the moments at which the protagonist, Louise Mallard, hears that her husband is dead, then discovers that he is alive after all
William the conqueror was a Norman king of England, and he was known among his own people as WIlliame I (Normans did not use surnames, so he was known as William the Bastar d, but rather to the Saxons).