items_packs and resource_pack

What are these folders?

items_packs

is a folder which contains a set of folders, each one of these folders contains separated and organized .yml files which contains items behaviour, setting, enchant, recipe, loot... everything that is related to items creation, recipes and behaviours. You can see that each folder name in items_packs is the namespace of the items it contains.

Namespace is also set inside the .yml file itself, don't forget to set it in the top part:

info:
  namespace: example

resource_pack

is a folder which contains an automatically generated resourcepack. You can see this folder and items_packs as a very similar thing, but resource_pack folder contains the "graphical" part of all items (and also sounds, GUIs...). (items_packs contains the behaviours and settings of the items) This folder is automatically compressed as a file named pack.zip, the one that will be sent to players.

Important

Remember to always maintain order in resource_pack folder, don't paste things randomly, don't create too many subfolders, don't leave unused textures/models or it will be difficult to find out errors and mistakes. Remember that please.