Rule 706.8 · Rolling a Die
One card (Centaur of Attention) has an ability that instructs a player to roll dice and “store” those results on it and another ability that allows a player to reroll any number of those results.
Subrules
To store the result of a die roll on a permanent means to note both the kind of die rolled and the result of that roll. That noted information is considered a “stored result” of that permanent, and the result is the “value” of that stored result.
To reroll one or more stored results of a permanent, roll one of the kind of die noted for each of them. If one kind of die is noted for more than one of those results, roll that many of that kind of die. The results you rerolled stop being stored results, and you store the results of each of the new die rolls on that permanent.