The phrase "to put one's life on the line" means to risk one's own safety or even one's life in order to do something. It is a metaphorical expression that suggests a person has gone beyond their normal limits and taken a significant chance with their well-being for the sake of achieving a goal, completing an important task, or helping others. The phrase can be used both positively (to describe someone who bravely fights for what they believe in) or negatively (to describe someone who recklessly endangers themselves).