Maror comes from the hebrew word for bitter. The maror symbolizes the bitterness of slavery in Egypt. Only certain vegitables are to be used to make maror such as: horseradish and romaine lettuce (the most common vegitables used). The blessing for the maror is

ברוך אתה ה אלוהינו מלך העולם אשר קידשנו במצוותיו וציוונו על אכילת מרור

Ba-ruch a-tah A-do-nai, E-lo-hey-nu Me-lech ha-o-lam, A-sher ki-d’-sha-nu b’-mitz-vo-tav, v’-tzi-va-nu Al a-chilat ma-ror.

Some traditions include putting the maror on lettuce and making a sandwich or dipping the maror in charoset.

Question: Why do we have Maror in the seder?