For the first ability, you roll a die that isn’t already in the game. That die will be used to represent the Die creature you create.

Die creatures have an inherent ability to be rerolled at any time, not just when they were initially rolled. Pippa’s second ability can reroll them, as well as effects of cards like Wall of Fortune.

The Die creature you create enters the battlefield.

Pippa’s second ability can be used to reroll any die roll, not just yours. If you reroll another player’s roll, the result still counts for whatever instructed them to roll a die, but abilities you control that trigger “whenever you roll a die” will trigger.