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

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

You can also get Orbs from Chests, the PvP Arenas, collections, and Event Islands, or by Recalling your dragons.

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