The Mirror is one of the most unique spells in Clash Royale. It can copy your previous deployed card with a cost of additional +1 elixir. The level of the mirrored card depends on the Mirror’s level.
The main use of Mirror is to copy strong cards that you will deploy. You can deploy first a Minion Horde and expect to be countered by Arrows, then use Mirror to send another Minion Horde, leaving your opponent unable to counter it again with the same card.