Apple File System (APFS) a new and reengineered file system for iOS, macOS, tvOS, and watchOS. APFS was announced at WWDC in 2016 as a replacement for the old system, also known as Mac OS Extended or HFS+, which they have been using since the early days of macOS.
With the release of the new macOS High Sierra, APFS will be the new default file system for every Mac computer with all‑flash internal storage. This new file system will provide better encryption, and faster directory sizing, and as well space sharing and management. For example, APFS will make simple tasks like coping or duplicating files happen instantly, compared to the old file system.
Note that APFS is currently being used on iOS (iOS 10.3), as well as tvOS (tvOS 10.2), and watchOS (watchOS 3.2).
For in-depth details about APFS, click here to visit Apple Developer Website.