AssetPack

An AssetPack is a collection of all assets needed by your addon. It's pretty much just your addon's resource pack. However, the format differs a lot from a vanilla resource pack.
To start off, create an assets directory in your addon's resources folder.

Asset Index Files

Depending on what you want to do, you can create a file called materials.json for custom items and blocks, guis.json for custom GUIs, and/or armor.json for custom armor textures. These files are called asset index files, as they're an index of the assets that you're adding to the game.