The maror is the bitter herb eaten at the seder, and it is one of the few foods the Torah itself commands for the night, its bitterness deliberately tasted as part of the retelling.
A commanded taste
The Torah commands that the paschal offering be eaten with unleavened bread and bitter herbs, and the maror is the fulfilment of that command, eaten at the seder with a blessing, and the Haggadah gives its meaning plainly: it recalls the bitterness of the slavery of the Israelites in Egypt. The point of the maror is that the bitterness is actually tasted, so that the account of oppression is not only heard but felt on the tongue, which is characteristic of the way the seder turns the story into an experience of the senses.
What counts as bitter
The tradition discusses which vegetables may serve as maror, several bitter plants being acceptable, and in practice grated horseradish and romaine lettuce are the common choices, the lettuce being bitter in its root or when it grows old even where the leaf is mild. The maror is eaten on its own and again together with the unleavened bread in a set order, and it is dipped in the sweet paste, the charoset, so that the bitterness and the sweetness are taken together. The commanded bitterness of the maror thus works alongside the other symbolic foods to make the seder a retelling that is eaten as much as spoken.