The Player Value Index is a system used by GGPoker to determine how the rake contribution by every individual player is calculated. Players are set in certain value tiers based on their overall record — the amount of play, deposits, withdrawals, etc.
With this system, casual players get more rewards as their rake contribution is calculated at a higher value. Conversely, winning players receive fewer Fish Buffet points for the same amount of rake paid.
The PVI system is a bit controversial, especially since no one knows the exact details of how everything works. However, you need to be aware of it when playing at GGPoker as it can ultimately influence your rakeback.