The vegetable dipped in salt water on the Seder plate is called karpas.
Karpas is usually parsley, celery, or another non-bitter vegetable. It is eaten near the beginning of the Passover Seder, after the blessing over wine and before the main meal. The unusual early dipping is designed to prompt children to ask why this night differs from other nights, supporting the Seder’s question-and-answer structure.
Salt water is commonly understood as a symbol of the tears shed by the Israelites during slavery in Egypt. Some interpretations connect the dipping to Joseph’s tunic, which his brothers dipped in blood, or to other stages in the Israelites’ descent into Egyptian bondage. The custom is not identical in every Jewish community: vinegar is used in some Sephardic and Persian traditions.
Karpas is easy to confuse with maror, the bitter herb eaten later in the ritual. Charoset is the sweet fruit-and-nut mixture, while beitzah is the egg. Karpas comes from a Persian word associated with celery.