Datapacks

In 1.13, Mojang added datapacks to the base game. They allow for the modification of the files for logical servers through the data directory. This includes advancements, loot_tables, structures, recipes, tags, etc. Forge, and your mod, can also have datapacks. Any user can therefore modify all the recipes, loot tables, and other data defined within this directory.

Creating a Datapack

Datapacks are stored within the data directory within your project's resources. Your mod can have multiple data domains, since you can add or modify already existing datapacks, like vanilla's, forge's, or another mod's. You can then follow the steps found here to create any datapack.

Additional reading: Resource Locations