1. To maintain a fragile balance between one extreme and another. i.e.: good and evil, sanity and insanity, decency and decadence, etc.
2. To behave; to abide by the the law and/or to abide by moral standards; to walk a straight path of decency by following the rules; to "walk the straight and narrow."
The latter definition is that applied in the song "Walk the Line" by Johnny Cash.
1. I walk the line between good and evil.
2. I used to be a drunken thug, but now I walk the line.
If a man has to emphasis many times that he'll loyal to his wife then mostly likely he won't. We'll never remind ourselves to breath as this is natural.
Thank you for all the above explanations. They have answered my puzzle.