Sometimes the rules allow you a 're-roll' of the dice. This is exactly as it sounds - pick up the dice you wish to re-roll and roll them again. The second score counts, even if it is a worse result than the first, and no dice roll can be re-rolled more than once, regardless of the source of the re-roll.
Note that re-rolling a 2D6 roll means picking up both dice and re-rolling them both, not just one of the dice. For 3D6 this would be three dice, 4D6 would be four dice, and so on.