What is a Second Seder?
It is a seder held on the Second Night of Passover, sometimes on a subsequent night.
It is a seder that can (but doesn't have to) depart from the traditional service.
It is a seder that various groups have embraced to include their own perspectives.
Arhur Waskow writes: "Let us indeed celebrate the first night of Passover as families, whether families of blood or of free communal choice. But on the second night, let us face the Pharaohs." (Source: The Freedom Seder: A New Haggadah for Passover)