Maror (מָרוֹר mārôr) also spelled Marror refers to the bitter herbs that are eaten at the Passover Seder in keeping with the Torah commandment: "with bitter herbs they shall eat it." (Exodus 12:8). The word derives from the Hebrew word mar (מר — "bitter"). According to the Haggadah, the traditional text which is recited at the Seder and which defines the Seder's form and customs, the maror symbolizes the bitterness of slavery in Egypt. The following verse from the Torah underscores that symbolism: "And they embittered (ve-yimareru וימררו) their lives with hard labor, with mortar and with bricks and with all manner of labor in the field; any labor that they made them do was with hard labor" (Exodus 1:14).
Place in the Seder
Maror is placed at two places on the traditional Passover Seder Plate. The first place is in the center, called Maror, this maror is used to fulfill the main requirement to eat maror at the Seder. The second place is at the bottom of the plate and is called chazeret (Hebrew: חזרת). This is used for the requirement, called Korech, when the maror is eaten together with matzo. There are various customs about the kinds of maror placed at each location.
During the Seder, each participant makes a special blessing over the maror and eats it. It is first dipped into the charoset—a brown, pebbly mixture which symbolizes the mortar with which the Israelites bound bricks for the Egyptians. The excess charoset is then shaken off and the maror is eaten. The Halakha (Jewish Law) prescribes the minimum amount of maror that should be eaten to fulfill the mitzvah (a kazayis or kayazit, literally meaning the mass of an olive ) and the amount of time in which it should be consumed.
The most commonly used vegetables used as bitter herbs are horseradish and romaine lettuce. Romaine lettuce is not initially bitter but becomes so after the first taste, which is symbolic of the experience of the Jews in Egypt.  Other suitable vegetables include endive and charchavinah (variously identified as a vine growing around palms, a type of thistle or a type of acacia), both of which are mentioned explicitly in the Mishna (Pesachim 2:6). Some Sephardic families use green onions or curly parsley. No flavorings of any kind may be added. The maror may not be preserved nor cooked. Ordinary jarred horseradish is unacceptable since it is soaked in vinegar. According to traditional Jewish authorities, romaine lettuce (chazeret) is the preferred species of maror. Romaine, endive, and dandelion are often eaten as maror because they are more palatable than horseradish.