Ezio was told, when he was in his 40s, that man named Desmond is the person who needed to here all the information he was learning, about the sun destroying the world. But that'd be retarded because Desmond isn't there. Fortunately, a device called an animus lets you relive the memories of your ancestors through their encoded DNA, which is inside of you. Ezio Auditore da Firenze, that means of Florence for you uneducated plebs, is one of Desmond's ancestors, and his ancestors in turn where able to predict Desmond's birth thousands of years later with Mathmatical power. So they left him a message in the 1400s and to be seen in 2012 in the most complicated way possible.
As for what Joker means, it could be a lot of things, from how the Assassins are constantly being shit on, to Assassin's Creed III being the absolute worst way to end Desmond's story. It has lots of meanings.