The Backpack allows a player to store a card for later use. The player places one of their Action Cards under the Backpack tile before filling their cards at the end of their turn.
Later, the player can play the Action Card from their Backpack instead of playing one from the board. When the player does this, the Backpack is discarded.
Note that the Backpack does not allow the player to play an extra card. On their turn, players may simply play the card from their Backpack instead of one from the board. It can be considered it as a temporary "4th card space".