What does the food on a Seder plate symbolize?

These are the main items on a Seder plate:

Maror (מרור), or other bitter herbs, symbolizes the bitterness of the Hebrews' enslavement in Egypt.

Charoset (חרוסת), a sweet mixture of apples and nuts that is usually mixed with red wine and cinnamon. Charoset represents the mortar which was used by the Hebrews to build storehouses and cities during enslavement. The Maror is dipped in the Charoset to counter the Maror's sharpness. It is also symbolic of how the bitterness of slavery actually manifested, i.e. building of massive Egyptian structures.

Karpas (כרפס), a vegetable such as parsley, which is dipped in salt water or vinegar to represent the tears shed by the Hebrews during slavery (an alternative explanation is that Karpas symbolizes Joseph's tunic which was dipped in blood by his brothers to convince their father that he had been killed by wild beasts, as in the Torah/Old Testament story). Used as an appetizer to induce the children to ask questions. Dipping is a sign of freedom, yet dipping in salt water reminds us of the bitterness of the slavery. Contradictions that cause one to ask, "why is this night different than other nights?"

Z'roa (זרוע), a roasted lamb's shankbone, It symbolizes the Korban Pesach (קרבן פסח) (Passover sacrifice) that took place in the Temple in Jerusalem while it still stood. The Jewish people were required to have a sacrifice on the 14th day of Nissan and eaten the night of the 15th when the temple stood. It is symbolic of the offering of the lamb they killed and it's blood that they put on the doorposts so the angel of death passed over their houses.

Beitzah (ביצה), a roasted egg, symbolizes the Korban Chagigah (קרבן חגיגה) Temple sacrifice (an alternative is that the ovoid shape of the egg represents the cyclical nature of life and the Universe). It represents the new life in which they would have in the promised land (Israel) and the reinvention of the Jewish identity.

Matzoh, (מצה) symbolic of the unleavened bread, that the Israelites consumed when they left Egypt in a hurry and did not have sufficient time to let it rise, eaten while crossing the desert.

The Seder Plate is linked to Pesach (פסח), or Passover in English, which concerns the story of Moses.