For those who, like me, don't follow developments in this area closely:

APFS is Apple's new filesystem, intended to be used across all of their devices (Macs, iOS, watchOS, etc). For Macs, it will be replacing HFS+, a direct descendant of the Hierarchical Filing System which has been used since 1985. (The original Macintosh Filing System, from 1984, was suitable only for floppy disks; among other things, it was a flat filing system and didn't really support folders.) The main improvements HFS+ made over HFS were support for larger files and Unicode; Apple has since added some more features but the core filesystem remains a 30-year-old format which was designed for early hard disks.