NSFileManager is Foundation's high-level API for working with file systems. It abstracts Unix and Finder internals, providing a convenient way to create, read, move, copy, and delete files & directories on local or networked drives, as well as iCloud ubiquitous containers.
File systems are a complex topic, with decades of history, vestigial complexities, and idiosyncrasies, and is well outside the scope of a single article. And since most applications don't often interact with the file system much beyond simple file operations, one can get away with only knowing the basics.