You need a certain number of Orbs of the specific dragon you want to Empower.

Recalling your dragons back to the Dragonverse will give you Orbs of that dragon.

You can also get Orbs from Chests, the PvP Arenas, collections, and Event Islands.

You can also use Joker Orbs if you are missing some Orbs to Empower your dragon; the Joker Orbs and the dragon rarity must be the same.