Original Resin in Genshin Impact is like the "stamina" system of the game. You can use these to get certain items that are core to a character's progression like increasing a character's max level/weapon, or levelling up talent skills.
Resins regenerate over time, at a rate of 8 minutes per resin.
You can recover 60 Original Resin by using Primogems up to 6 times per day. The first time will cost 50, then 100, 100, 150, 200, 200.
The other way to gain some resin is by using Fragile Resins. Use this by going to your inventory, and go to the "precious items" tab. Select the fragile resin and click use. You will recover it by 60 points, and you can go over the limit by doing this like from 85/120 to 145/120.
Condensed Resin is a new item added into the game after version 1.1 update. You can store your original resin using Crystal Cores that you get from crystal butterflies, to make a Condensed Resin.
Once you get the blueprint, you will need to use it in your inventory to learn it so you can craft it at the crafting table.
After using the blueprint, you can now craft it at the crafting table. It costs 40 Original Resin, 1 Crystal Core, and 100 Mora to make 1. Once you turn it into Condensed Resin, you are not able to use it to regain 40 Original Resin back. The only way to use it is for domains, or ley lines.
How and where to use resin
There are 4 ways to use your resins: Ley Lines and/or Domains costs 20 resins, bosses like Hypostasis and Regisvines costs 40 resins, and weekly boss costs 60.
Condensed Resin can only be used on Ley Lines or Domains. Since Condensed Resin costs 40 Original to make 1, using a Condensed Resin when picking up rewards will give you double the rewards. However, the 2 sets are separate and still randomized.
Ley Lines and Domains
Once you reach a certain Adventure Rank, you will be able to start doing Ley Lines. The rewards from these Ley Lines scales with your Adventure Rank. They will appear on your map, and you can do them whenever. Once you go to it, activate it, and some monsters will appear.
You can get some rewards from the enemy drops, but getting the ones shown from the adventure handbook requires 20 resin. Once you claim the rewards using Resin, the Ley Line will appear again but in a different place, so you don't have to worry about them expiring, limited, or anything like that. However, they will not respawn until the next server reset if you do not pick up the rewards.
There are 2 different Ley Lines that you can do:
Blossom of Wealth
Blossom of Wealth is for Mora, which is the gold and standard currency of the game to buy things from shops. There's also friendship points and 100 adventure rank exp, but the other ley line has it as well, so it's not exclusive.
Blossom of Revelation mainly gives items for you to level up your characters like Wanderer's Advice or Adventurer's Experience. The amount that you get varies.
Domains are for getting items to ascend your weapons, increase talent levels, or to get domain specific artifacts. Each domain costs 20 resin to pick up the rewards. You can view available domains by going to your adventurer handbook (F1 on pc), then go to the domains tab. Domain rewards changes each day, so you'll have to check which day drops what, by tapping the "?" icon on the right when viewing the domain(s).
These are mini bosses that you can find on the world map and fight. There are different elemental Hypostasis and drops different elemental items and equipment. You will need 40 resin if you want to pick up the rewards. These bosses are viewable from your adventurer's handbook and in the bosses section. Simply tap the "navigate" button to find where they are on the map.
You can only obtain the rewards for these once a week, and they reset every monday. These cost 60 resins to claim the rewards.
The main quest for Dvalin is available once you reach Adventurer Rank 18, but you need to be at Adventurer Rank 25 to do the fight again and get rewards. There are up to 6 difficulty levels, and each difficulty level is separated by 5 Adventurer Rank (25,30,35,40).