If you're missing some Orbs to Summon or Empower a dragon, you can use Joker Orbs as a replacement.
The Joker Orbs must be of the same rarity as the dragon you wish to Summon or Empower.
Use the 'i' info button in the top right of the Summon or Empower screens to see how many Joker Orbs you currently own.
Please note that there are restrictions on how many Joker Orbs can be used. You still need to own a certain amount of Orbs for the dragon you wish to Summon or Empower. These restrictions can change depending on the Empower level or rarity of the dragon.