Lanolin is a mod that adds (surprise) lanolin. A resource obtained from smelting sheep's wool which can be applied to armor to make it water resistant.
Water Resistance is a armor bonus that extends how long a wearer can hold their breath, but it wears off. A suit of fully water resistant armor increases the time you can hold your breath by over a minute before it has to be reapplied.
To apply lanolin to armor, simply craft them together (can craft multiple at a time). Crafting with lanolin preserves NBT data on the item, so you don't lose enchantments.