Passover is an annual eight-day long holiday in the Jewish religion. Unlike most Jewish holidays which are mostly observed in the synagogue, Passover is typically celebrated in the home with a large ritualized dinner called a seder. While there is certainly a lot of food involved in a seder, that is not the main focus of the event. Instead, as Hannah's mother reminds her daughter, Passover is about remembering the past.
Specifically, Passover tells the story of the Jewish people's exodus from Egypt. One significant part of the seder involves repetition of the line from Deuteronomy 15:5, "Remember that you were slaves in the land of Egypt . . . " Throughout the seder, the participants recall the tribulations of the Jews in Egypt and their flight from bondage. Many modern seders, take this a step further and use it as a vehicle to form empathy with oppressed peoples all over the world and throughout history. For Hannah's grandparents, the Passover seder is an important time for remembering the people who they lost in the Holocaust.