To be able to walk along a straight line is a test in order to show that the person is sober (he did not recently use too much alcohol). This is the reality; in an extended meaning it means that somebody (capable of 'walking the line') keeps at large from bad habits [and maybe is also caring for his family]. At the time the song has been written, the electronic devices to measure alcohol level in the blood from the breath did not exist, so the police used these kind of 'down to earth' tests when questioning vehicle drivers.

It means he is at the verge, at the limit. He lives dangerously.

Wow. so many answers... thanks a lot! they were really very helpful. Now I can play the song at school and explain it. Esther

Jonny Cash wrote the song 'Walk the line" for his 1st wife Vivian and it means to be faithful to her while he was traveling all over America singing.

Johnny has changed the idiom 'toe the line' to 'walk the line' in this song. Toe the line: to do what you are ordered or expected to do Not everyone was happy with the plan, but most of us toed the line. He means 'remain true' to his lover. He will not stray(cheat), he will do what is right. I find it very, very easy to be true(will not cheat) I find myself alone when each day is through Yes, I'll admit I'm a fool for you Because you're mine, I walk the line