Hannah is a Jew being sent to a concentration camp during the Holocaust. She has been handled roughly, starved, and crammed into a boxcar with tens of other people. So, when she is finally handed water, even though it's filthy trough water, she thinks it's sweet because at least there is water to quench her thirst; in other words, it's better than no water, and since she is so thirsty and dehydrated, this water is like heaven.
