these are used to define spaces, and must always be specified when choosing the path to a model, for example minecraft:item/diamond which will reference assets/minecraft/models/item/diamond.json
pack.mcmeta
What is this, this is to define the version and the description in which the resourcepack will work, which is located in the main path of the resourcepack
where you can find all the mcmeta by versions is here in "List of resource pack formats", and this is located in
overlays
these are used to apply textures and anything else to a given version of minecraft, it is used to add compatibility to multiple versions without breaking
And these need to be specified in the pack.mcmeta to work.
You can use both formats to support old and current versions, example:
pack.png
This is to define the image that will be seen when selecting the resourcepack, generally it is 64x64.