MoneyPak is a card you can buy at stores in the United States. You add money to the card for a $5 fee. The moneypak card can then be redeemed on your(or the seller's) paypal. After the moneypak card is redeemed, it is permanent. So there can be no chargeback or anything like that.
So you use a moneypak card to add money to your paypal.