To meet a particular standard; to conform strictly to a rule. The term comes from track, when the runners in a race line up with their toes placed on the starting line or mark.

What is the origin of the phrase toe the line?

The most likely origin of the term goes back to the wooden decked ships of the Royal Navy during the late 17th or early 18th century. Barefooted seamen had to stand at attention for inspection and had to line up on deck along the seams of the wooden planks, hence to "toe the line".

Is the expression in toe or in tow?

The correct version of this phrase is always toe the line. The phrase means “follow the rules or obey an authority.” “Tow the line” is a misspelling based on “toe” and “tow” sounding the same, but “tow the line” is always an error.