Michael Cohen, writing for TidBITS:

Among the tidbits Apple revealed to its developer audience at the recently completed Worldwide Developers Conference was a new file system for the whole range of its products. Dubbed “APFS” (an acronym that Apple doesn’t completely spell out even in its developer documentation), the file system is meant to replace HFS+, the file system that in turn replaced 1985’s HFS (Hierarchical File System) in 1998. (HFS+ has received numerous updates since 1998, so don’t get the impression that it’s completely obsolete.) A developer preview of APFS is baked into the forthcoming macOS Sierra and Apple says APFS will become the default file system in all of its operating systems — macOS, iOS, watchOS, and tvOS — by late 2017.

Nice high level walkthrough of a major unifying change coming to your favorite operating system.

As to the acronym APFS, the sense I get from an informal Twitter poll is that APFS stands for Apple File System. And, as Kirk McElhearn points out here, Apple uses the acronym APFS and the name Apple File System consistently, as if the acronym came from APple File System and not Apple Pxxx File System.