Minecraft wiki entry for Creating_resource_pack_add-ons, associated text: A UUID is an identifier that we will use to differentiate between our various add-ons. For this reason, UUIDs must be unique! If you don't change the UUID from pack to pack, your pack may not show up in game! But UUIDs are easy to generate! An online generator can be found here. Refresh the page when you need a new one.