Orbs unlock new gear that may or may not be lootable from normal questing. Orbs will always drop items that are within a certain range of your heroes’ levels, so you will never get a Paladin item that’s Level 1 if your Paladin is at Level 50.
Orbs are also one of the game’s IAP (in-app purchases). You can buy them with real money, but you can also earn them within the game by collecting Shards. Every 5 shards turn into an orb. You can earn shards by completing stages at 5 Gold stars, completing daily challenges, participating in events, and through daily rewards.