The Hold Card slot lets you "hold" a card to swap in at any time in place of the current "active" card, so it becomes the new active card.

For example, if you draw a King but are unable to use it right now, you can place the King in the Hold Card slot for later use when a Queen or Ace is revealed.

Or, save a Golden Tiki or Wild Card in the Hold Card slot to use it later.

Hold Cards are available on certain levels and sometimes can be given as a Perk Reward.