To store a result means to note the result, ya see. You can do it by keeping a die with that result on Centaur of Attention, or just by writing down the result. To reroll a noted result, roll a die with the same number of sides and replace the noted result with the new result.

You may reroll any results stored on Centaur of Attention, even those you chose not to reroll on previous turns.

To get the largest bonus from the last ability, you want all five dice to have the same result. It doesn't matter what result that is. The value of X is the largest number of dice that match, not what number they're showing. For example, if your five results are 1, 1, 3, 3, and 4, Centaur of Attention would get +2/+2.

Something in the game must tell you to roll a die. If you roll a die for any other reason (to simulate a coin flip, to choose pizza toppings, to create alternate timelines), that roll doesn't count.

If a die is rerolled, the original roll essentially never happened: it doesn't cause any abilities to trigger, and no effect that cares about die rolls will consider it.

If Centaur of Attention's enters-the-battlefield ability is copied or triggers multiple times somehow, it's possible to store more than five results on it. This is awesome. The last ability will consider all stored results and look for any that match each other.

Some effects may modify the result of a die roll. This may be part of the instruction to roll a die, or it may come from other cards. Anything that references the "result" of a die roll is looking for the result after these modifications.

If an ability triggers "whenever you roll a die," it will trigger whenever you roll any die, including the planar die. This is a change from previous Un- rules. Some abilities use the result to determine part of the effect. If you get a non-numerical result (currently just the planar die, but the future is long), that part of the effect won't do anything.

Each die is identified by the number of faces it has. A six-sided die is a die with six equally likely outcomes: 1, 2, 3, 4, 5, and 6. The roll must be fair. Although physical dice are recommended, digital substitutes are allowed except in cases where the physical die is required for the effect.

Results can be numbers not ordinarily possible on a six-sided die. Spells like Scooch can change the result to 0 or 7, for example.