It isn't defined super clearly in terms of how it relates to your character but how it works and how it works in the game is defined at the bottom of page 22.
Essentially composure is your ability to resist compulsions that result from hunger and works similarly to fate points from fate. When one of your rolls triggers a compulsion you can either accept the compulsion and act it out (possibly gaining a point of composure from the storyteller)
Acting out the compulsion rewards you with a point of Composure, at the Storytellers discretion.
or refuse it by spending a point of composure.
The only way to avoid this consequence is to spend Composure, allowing you to quash the unwanted impulse, at least temporarily.
Also the reason it has temporary values is on page 22 as well.
Just like Willpower, Composure is a spendable trait, with a current and a maximum value.