This page of the Genshin Impact provides information about Condensed Resin, how to unlock it, where to craft it and whether it is worth using.
Condensed Resin is an item not much different from Original Resin, a special currency. Condensed Resin is nothing more but an Original Resin with twice as much content. This means that the player will have to spend 40 Original Resins and 1 Crystal Core to craft this item.
This allows the player to use Condensed Resin in Domains and Ley Line Outcrops to get twice as much loot. Condensed Resin was created mainly for players who do not have much free time to farm artifacts, Mora or EXP materials. You can craft Condensed Resins at any alchemy table. You only need to have 1 Crystal Core and 40 Original Resins at your disposal.