What the hell is an 'Assassin's Creed' anyways?

It is what they call the Creed they follow "Nothing is true; everything is permitted"

It is explained in the games.

"To say that nothing is true, is to realize that the foundations of society are fragile, and that we must be shepherds of our own civilization. To say everything is permitted, Is to understand that we are the architects of our actions, and that we must live with their consequences, whether Glorious or tragic."

A creed is a statement of a system of beliefs.
Like some Christians believe that creeds are wrong, and say that "We have no creed but Christ" but that in of itself is a creed.

Exactly what byzantiumthrace says;
In the first game, there were three tenets mention at the core of the creed:

1. Stay your blade from the flesh of an innocent

2. Hide in plain sight

3. Never compromise the Brotherhood

The lines stated in the movie are spoken by Ezio in the games:

“Where other men blindly follow the truth, Remember, nothing is true.
Where other men are limited by morality or law, Remember, everything is permitted.
We work in the dark to serve the light.
We are assassins.”

This was an expansion of the "Nothing is true, everything is permitted" thing.