I am familiar with a Passover seder, but what is a Tu BiShvat seder?
Tu BiShvat (Hebrew for the 15th day of the Hebrew month of Shvat) is the new year of the trees.
The sixteenth- and seventeenth-century kabbalists (mystics) created this ritual, modeled after the Passover seder. They would gather for a multi-course meal with fruits that are associated with the Land of Israel. Commonly included foods include fruit from three different classifications: fruit with shells (oranges), fruits with pits (dates), and fruit that are entirely edible (raisins).