Karpas is a green vegetable, most often parsley. Karpas represents the initial flourishing of the Israelites during the first years in Egypt. At the end of the biblical book of Genesis, Joseph moves his family to Egypt, where he becomes the second-in-command to Pharaoh. Protected by Joseph’s exalted status, the family lives safely for several generations and proliferate greatly, becoming a great nation. The size of this growing population frightens the new Pharaoh, who enslaves the Israelites, lest they make war on Egypt. Even under slave conditions, the Israelites continue to reproduce, and Pharaoh eventually decrees that all baby boys be killed.

Eating karpas as the third step links our practice todayto ancient history. During the time of the temple,all formal dinners began with an appetizer. Karpas serves as an appetizer, to activate our hunger literally and figuratively for the rest of the seder.

In the course of the seder, we dip the karpas in salt water (Ashkenazi custom) or vinegar (Sephardi custom) in order to taste both the hope of new birth and the tears that the Israelite slaves shed over their condition.

Karpas also symbolizes the new spring. One of the names for Passover is Hag Ha-Aviv or the “holiday of spring.” Right around Passover the first buds emerge, and we look forward to the warmth and sense of possibility that accompany the beginning of spring.

haggadah Section: Karpas
Source: http://www.myjewishlearning.com/holidays/Jewish_Holidays/Passover/The_Seder/Seder_Plate_and_Table.shtml