1. When Brad lost his regular job, he had to take any temporary one that came along, and he never knew when he’d find another. He lived hand to mouth.
2. Some people in areas affected by drought live a hand-to-mouth existence. They collect what little food they can and never have enough to save some for another day.

The expression suggests that a person’s need is so immediate that what he collects in hand goes straight into his mouth and that there is no room for saving or planning ahead.