In Parshat Beshalach, פרק יב, פסוק ח, we find the Mitzvah of והגדת לבנך, telling your son about יציאת מצרים. The reason for this is that it was a pivotal event in Jewish History. It is particularly important to tell it on the night of the Seder, because on the table during the Seder, are two reminders of the story of Pesach. These visual features are:
a) Matzah, which was baked in haste by our ancestors when they were leaving Egypt.
b) Four cups of wine which symbolize the four expressions of redemption by G-d. Even poor people are required to drink these cups.
c)חרוסת -symbolizes the mortar with which our ancestors built Egyptian cities.
d)מרור -we are commanded in Parshat Behalotcha to eat Maror with the Matzah and Korban Pesach, but after the Beit HaMikdash was destroyed, we eat an equivalent to an olive of Maror as a sign of mourning for the destruction of the Beit HaMikdash.
e)אפיקומן -this is instead of the Korban Pesach. In the past the Korban Pesach would be eaten at the end of the meal to show that this is separate from the rest of the meal.
The Mishna (Pesachim 611a) teaches that if there is no child to tell the story to, one must tell it to his wife. If he doesn't have a wife, he must ask himself the questions and answer it himself. According to אור החיים, by telling the story to himself, he will become worthy enough to have his own children to tell it to. If one has no children, why must he tell it to himself out loud? The ספר החינוך says that he will get a greater understanding of it, and it will be more meaningful, if he hears it in addition to just thinking about it. According to Menachem ben Benjamin Recanti, he might be so influenced by the story, that he will enthusiastically tell it to others, and they will tell it to others, and so on.