What do ashes on Ash Wednesday signify?

The ashes are burnt from the palms of the past year's Palm Sunday and they represent the sin that we have and the death that we will face (as in "for dust thou art and unto dust thou shalt return" Genesis 3:19). That sounds really morbid, but it's a way to begin and prepare yourself for the season of Lent, remembering sins and the redemption that has been given to us all.