The entire story of the Haggadah is contained in the Seder plate (קערה ke'ara); everything on it symbolizes an aspect of Exodus:

• Zeroa (זרוֹע)a roasted bone, evokes the offering made at the Temple in ancient times.
• Beitza (ביצה‎), a boiled egg, symbolizes the circle of life and death.
• Maror (מָרוֹר), a bitter herb, reminds us of the bitterness of enslavement.
• Charoset (חֲרֽוֹסֶת), a mixture of fruit, nuts, wine and spices, represents the mortar our ancestors used to build the structures of Mitzrayim
• Karpas (כַּרְפַּס), a green vegetable, symbolizes hope and renewal.
• Chazeret (חזרת), the bitter herb for the “sandwich” we eat later, following the custom established by Hillel the Elder, as a reminder that our ancestors “ate matzah and bitter herbs together”

In addition to those 6 traditional foods, you'll notice that we also have chocolate and an orange:

Fair Trade certified chocolate is grown under standards that prohibit the use of forced labor. It is placed on the seder plate to remind us that forced labor is still with us today. The Fair Trade movement promotes economic partnerships based on equality, justice and sustainable environmental practices. We have a role in the process by making consumer choices that promote economic fairness for those who produce our products around the globe.

I'll let Susannah Heschel explain the orange...