Rerolling is a mechanical term. It is the act of taking Die that have already been rolled and rolling them again to generate different Results.

A Reroll allows you to pick up an amount of Dice from a Pool, as determined by the source of the Reroll, and roll them again for different Results.

You cannot Reroll Die 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.