Rabban Gamliel would say: Whoever does not discuss the following three things on Passover, has not fulfilled his obligation: the passover offering, matzah and maror.

The passover offering, which our ancestors ate during the time of the Holy Temple - for what reason did they do so? Because G‑d passed over their homes in Egypt, as it is written: "You shall say: It is a Passover-offering to the L‑rd, because He passed over the houses of the children of Israel in Egypt when He struck the Egyptians with a plague, and He saved our houses... "

This matzah that we eat - for what reason? Because the dough of our fathers did not have time to become leavened before the King of the kings of kings, the Holy One, blessed be He, revealed Himself to them and redeemed them. As it is written: "They baked matzah-cakes from the dough that they had brought out of Egypt, because it was not leavened; for they had been driven out of Egypt and could not delay, and they had also not prepared any provisions."

This maror (bitter herb) that we eat - for what reason? Because the Egyptians embittered our fathers' lives in Egypt, as it is written: "They made their lives bitter with hard work, with mortar and with bricks, and with all manner of work in the field; all their work which they made them serve with crushing labor."

-From the Passover Haggadah