It is customary to begin the meal with hard-boiled eggs flavored with salt water. This was the practice in Roman times. The egg has come to be symbolic of new growth, of new life, of hope. The roasted egg on the Seder plate has come to represent the ancient Temple service in Jerusalem, the holy city. 

haggadah Section: Shulchan Oreich
Source: A Passover Haggadah - CCAR