What is original resin?

This is the moon-like icon on the upper-right corner of your map. This is used to open reward chests from completing domains and bosses, and is replenished over time. You can also spend 50 primogems to replenish 60 resin, up to 6 times a day. This cost increases as you spend primogems to replenish the resins.

You can also use consumables called Fragile Resin which restores 60 resin. However, these are extremely rare and can be obtained in limited numbers as adventure rank rewards. Other than what we mentioned here, there's no other ways to expedite the natural replenishing rate.