It’s often misunderstood, even for experienced players and fans as to how far a player can run out of the baseline. The confusion is not so much due to the distance (three feet on either side) as much as where the “line” is.
The rule states that as soon as a fielder attempts to tag out a runner a path is formed between the runner and the base he is attempting to advance to. The runner has three feet on each side of the line that he must stay within.
Primarily the confusion enters the situation when people think the line is formed in a straight line from base-to-base, when for the purposes of this rule it is actually from runner-to-base. This means that technically a runner can lead off a base anywhere on the field he wants stand (although he must be careful not to stray too far from a base and get picked off).