Before they rebased onto UNIX, Mac files had two parts: a data fork and a resource fork. Data had the binary. Resource had strings, images, and other assets that the binary needed.

It was great for local portability: you could put the file wherever you wanted, and it would still be colocated with its stuff. It was bad for network portability, because a lot of transport layers didn't respect the forks.

So Apple migrated to bundles - magic folders that look like a single file in the Finder, but contain all the resources it might need.