Literally, “four species.” The Torah specifies four species to bring together on Sukkot. The four species are: lulav (branches of palm trees), etrog (citron), hadasim (myrtle branches), and aravot (willows) (Leviticus 23:40).
At the beginning of the Passover seder, the youngest person asks the Four Questions. Here's a handy guide to the questions in English and Hebrew, with a downloadable melody and printable version.
Don't be overwhelmed by hosting your own seder. Use this handy checklist to help you prepare!