In the military, for example, soldiers are made to stand in a perfect line, and they are inspected by higher ranking officers so that their stance is perfect... not a toe out of line, in other words. In the movie, "Full Metal Jacket" you can see soldiers lining up their feet, whether they are in socks, or in boots and full gear, in a perfect line. In this movie there is also graphic violence so be careful. Hope this helps.... 8]

The idiom is usually expressed as "toe the line". It means to conform or follow the accepted ideas and behavior. Negated, it is often given as "out of line".