What is the meaning of horseradish in the seder dish?

THE MAROR or "Bitter Herbs" (usually horseradish) is placed in the middle of the Seder dish and symbolises the Jew's' bitter suffering under the Egyptian yoke. directly below is the chazoreth, another piece of bitter herbs (or radish) commemorating the custom of eating maror sandwiched between two pieces of the matzoh.

Hope i helped! If you didn't understand here is the simple version: symbolises the Jew's' bitter suffering under the Egyptian yoke. :)