PXP stands for Pookyball Experience Points.
For each successful prediction; the Pookyball used to make that prediction will gain Experience points.
Experience Points (PXP) are calculated by multiplying Boosted Points and the Rarity level of the digital collectible Pookyball.
$$ PXP = Boosted Points * Rarity Multiplier $$
PXP are awarded to the Pookyball used for a prediction, at the moment when the player claims Leaderboard rewards.
PXP allows players to Level-Up Pookyballs.
Rare, Epic and Legendary Pookyballs have more Levels, but their higher earning of PXP means they'll be, on average, higher level than a lower tier ball.