Do Over is a mechanical term. It is a unique method of generating different Results from an already rolled Dice Pool.
A Do Over allows you to pick up an amount of Dice from a Pool, as determined by the source of the Do Over, and roll them again for different Results.
In addition a Do Over allows you to roll Dice that have already been Rerolled.
Please Note: Abilities such as the Lucky Break Action allow for a Do Over, which is distinct from a Reroll.