What does a Christmas poinsettias represent?

First of all, technically it isn't a flower and the red parts of it everyone judges to be petals are actually leaves.

Secondly, the poinsettia was introduced by the first ambassador of Mexico, Joel Roberts Poinsett. When he brought it to Mexico the Mexicans were symbolic to the star of David (star of Bethlehem, and that is how the Poinsettia became associated with Xmas.

What Joel Poinsett announced was the Poinsettia was actually to represent 'Courage in the Darkness'