When you draw your starting hand each combat, you draw two more cards. The bonus cards are drawn along with your starting hand, generally five.
Bag of Preparation is a common relic which functions identically to Ring of the Snake, the starting relic of The Silent class. The two relics stack, so you begin the turn with nine cards.
If you also have the Gambling Chip, you may mulligan the additional cards.
The card art shows the same bag from Prepared.