A piece of roasted meat represents the lamb that was the special Paschal sacrifice on the eve of theexodus from Egypt, and annually, on the afternoon before Passover, in the Holy Temple.
Bitter herbs (maror) remind us of the bitterness of the slavery of our forefathers in Egypt.
The lettuce symbolizes the bitter enslavement of our fathers in Egypt. The leaves of romaine lettuce are not bitter, but the stem, when left to grow in the ground, turns hard and bitter.
So it was with our enslavement in Egypt. At first the deceitful approach of Pharaoh was soft and sensible, and the work was done voluntarily and even for pay. Gradually, it evolved into forced and cruel labor
The word karpas is a reminder of the sale of Joseph into slavery, an act that led to the descent of the Israelite people into Egypt. The karpas itself is, therefore, an appropriate introduction to the events recounted in the Haggadah [narrative] -- the enslavement of the Israelites and their ultimate redemption. The word karpas [כרפס] appears in the Bible once, in the phrase ותכלת כרפס חור [hur karpas u'tekhelet] (Est. 1:6). In this context it means "a fine linen," using a word borrowed from the Sanskrit or Persian kirpas
A hard-boiled egg represents the holiday offering brought in the days of the Holy Temple. The meat of this animal constituted the main part of the Passover meal.
A mixture of apples, nuts and wine which resembles the mortar and brick made by the Jews when they toiled for Pharaoh