Clash Royale now features a new in-game currency called "Elite Wild Cards," which is your key to unlock Level 15 for each card.
In the original progression system that is still active from Level 1 to Level 14, you first need to collect a certain amount of cards based on the unit's rarity
and then spend a certain amount of Gold on upgrading your unit to a new level.
However, with Elite Wild Cards, the upgrade system from Level 14 to Level 15 has been simplified.
No matter what your card's rarity is, you are going to need 50,000 Elite Wild Cards to upgrade it to the Elite Level.
Once you have the required amount, you won't need to pay any Gold, as your card will automatically upgrade to Level 15.
As you can guess, Elite Wild Cards are universal and don't belong to a specific unit.