On your mark, get set, go! When you stand ready for a race with your toes on the starting line, you literally toe the line.
In fact, people used to say, "Toe your mark, get set, go!" These days, the expression toe the line is more commonly used to refer to doing what's expected of you, or conforming. If you constantly misbehave in class, your teacher might tell you that you need to start toeing the line. This phrase can also mean being right up against the line between two different things. If your short story toes the line between fantasy and science fiction, then it probably includes elements of both those genres but doesn't neatly align with either one. Any way you use it, the idiom toe the line always uses toe as in the one at the end of your foot.
These examples toe the line of good usage:
Your decision creates the difficult impression that only those who "toe the line" will be rewarded, and anyone who dares express a political opinion outside of the consensus will be punished. (Washington Times)
Mann's photographs of her children toe the line between art and reality. (Los Angeles Times)
No one will know the difference if you say it out loud, but tow the line is wrong. Tow means to drag or pull. While you can theoretically pull a line of rope or cord, doing so doesn't really capture the sense of this expression, which is fundamentally about alignment: either with a starting point or boundary, or with a standard or norm. Tow the line is a common mistake, as you can see below:
Throughout the day, his campaign fought to control its rambunctious delegations, sending texts to supporters urging them to tow the line. (Time)
In reality, ambiverts, those people that tow the line between extroverts and introverts, tend to be the most successful salespeople. (Forbes)
To remember this expression, imagine all the ways to literally toe the line: starting a foot race, lining up in gym class, staying in bounds in a game, or standing at attention in the military. When you toe the line, you're doing the right thing.