It celebrates the freedom of the Jewish people from slavery over 3,000 years ago. A big feast takes place and over the course of the night you will have four cups of wine, vegetables dipped in saltwater, matzah, bitter herbs and then a festive meal. ceremonial foods are arranged on a seder place.
The Jewish Sedar is celebrated every year after nightfall on the first night of Passover (and the second night if you're outside of Israel)
At the Seder, everyone is expected to feel as if they were going out Egypt. Somebody than reads aloud Abraham, Isaac and Jacob, recalling the suffering of the Jewish people.
The seder begins with the recitation Kiddush, proclaiming the holiness of the holiday. After Kiddush four cups of wine drank, the reason four cups are drunk is remembering the pharaohs 4 expressions. The next section of the seder consists of Urchatz (the washing) we complete this section by washing our hands in ritually prescribed manner.
We then begin the next part of our Seder, Yachatz. We do Yachatz by breaking the matzah (a bread like a cracker) the "bread of poverty" then remains visible as we tell the story of Exodus and is eaten shortly after. After Yachatz is concluded we begin Maggid (the readings)
After Urchatz we begin the Karpas. In the Karpas, we take a small piece of vegetable (often an onion or boiled potato) and dip it into salt water and eat it (after resiting the blessing over vegetables)