What is "stagger" in a 400 meter track race?

The "stagger" in a 400 meter track race allows athletes in outer lanes to start further ahead than those on the inside lanes, according to Maths is Fun. This ensures all runners complete the same distance during the race.

For races in which each runner has to stay in his lane, the semicircles at the two ends of a 400 meter track normally force outside lane runners to travel further distance. However, with a staggered start every athlete is provided an equal chance to win because no single runner has an advantage. It is believed that those in the inside lanes gain an advantage by seeing the rest of the field ahead of them, but this is balanced out by these runners plotting a tighter curve.