What are Dragonbone, Dragonglass and Dragonfire keys?

They're the three key types used to open Hatchery Chests:

• Dragonbone Key
• Dragonglass Key
• Dragonfire Key

Without keys, you can also open chests directly with gold.

Good news: all three keys are permanent — they don't reset between Reigns, like gold and Relics. So you can stockpile them calmly from one Reign to the next and do a big batch of openings when a favorable chest event shows up.

You get them through events, progression rewards, and you can also buy them at the Market (with Relic Shards).