What is Passover?

The Seder is the special meal that celebrates Passover

Pass·o·ver

   [pas-oh-ver, pahs-]  Show IPA

noun

1.

Also called Pesach, Pesah. a Jewish festival that commemorates the exodus of the Jews from Egypt and is marked chiefly by the Seder ritual and the eating of matzoth. It begins on the 14th day of Nisan and is celebrated for eight days by Orthodox and Conservative Jews outside of Israel and for seven days by Reform Jews and Jews in Israel.

2.

(lowercase) paschal lamb (def. 1).

Origin: 
1520–30; noun use of verb phrase pass over, as translation of Hebrew pesaḥ

Dictionary.com Unabridged Based on the Random House Dictionary, © Random House, Inc. 2013. 

Passover is one of the most important religious holidays in the Jewish faith. Its dates overlap  Easter.  The story of Moses leading the Hebrew slaves  out of Egypt is told in the Book of Exodus, Chapter 12 in the Hebrew Bible (the Torah) 

See here for a short summary of Passover traditions.