The karpas is the vegetable dipped in salt water near the start of the seder, before the telling begins, and its function is structural rather than symbolic: it exists to make a child ask why.
An early oddity
Early in the seder a green vegetable is taken, dipped in salt water, blessed and eaten, an unusual act placed before the meal and before the main narrative, and its oddity is the point. The seder is built to provoke the questions of the young, and the eating of a vegetable dipped in salt water at the outset, out of the ordinary order of a meal, is one of the departures from custom designed to make a child notice and ask, which opens the way to the telling. Where the other foods carry meanings the Haggadah explains, the karpas is chiefly a device to prompt the question.
Later meanings
Interpretations have been attached to it, the green vegetable read as a sign of spring and of new life and the salt water as the tears of the enslaved, so that the dip is often explained as joining the freshness of the season to the grief of bondage, but these senses are secondary to its structural role at the head of the seder. The karpas thus shows how the seder works: an action is placed to arrest attention and raise a question, and meaning gathers around it afterward. It is a small part of the observance, but it opens the sequence that the whole evening then unfolds.