When you have collected enough duplicates of the same card and have the required amount of Credits you can combine the cards together to get a more powerful version of that card.
Common:
These cards can be found all over, they have gray backgrounds.
Rare:
Not as hard to find as Epic cards, but you still won't find any in lower tier bronze packs.
Epic:
These cards are quite powerful and rarely in packs in the lower tiers.
Unique:
These cards will have an orange background and are tied to a specific leader, meaning they can only be placed in decks with that leader. In the image above Director Orson Krennic is my leader and Death Squad Leader is his unique card.
Legendary:
These are leader cards and are the rarest cards in the game.