In some situations, the rules allow you to pick up and re-roll a dice. This is exactly what it sounds like – pick up the dice you wish to re-roll, and roll it again. The second score counts, even if it means a worse result than the first, and no single dice can normally be re-rolled more than once, regardless of the source of the re-roll. If you re-roll a single 2D6 or 3D6 roll, you must re-roll all of the dice and not just some of them, unless the rule granting the re-roll specifies otherwise.
If a model is granted a re-roll from a special rule or similar while the enemy might have a special rule or similar that forces successful rolls to be re-rolled, they cancel each other out and no re-rolls are made for as long as both special rules are in effect.