Crafting is a way to upgrade cards without finding them in Packs.
You gain Ingots from playing games and picking cards you already have in packs.
Crafting is available from the main menu, and lets you browse your cards per faction. Ingots can be used to improve a card's level, just as if it was picked in a pack. Note that the ingot cost is per pick, and cards needs several picks to be improved. It's not possible to craft new cards, or improve Hero-cards.