What are wild cards?

When leveling up, I've opened several packs that contained wild cards of various rarities (rare, epic, etc), however I don't see them under my card collection. What are these cards, where can I find them in my collection, and what can they be used for?

Solution 1:

From the Legends of Runeterra FAQ page

Wildcards can be traded for a card of corresponding rarity. That means that a Rare Wildcard can only be exchanged for, well, a Rare Card! We’ve listed the card rarities below:

• Common
• Rare
• Epic
• Champion

They can be used by right clicking the card you want to exchange a wild card for, and clicking the Wild Card button.