One of the main features of macOS High Sierra is the new file system of APFS. Apple File System is a new, modern file system optimized for working with Flash / SSD drives. It includes the functionality of strong encryption, copy-on-write, space sharing, cloning files and folders, creating snapshots. As part of the working with the Apple File System (APFS) volumes, it may be necessary to decrypt a boot drive using APFS’s native encryption in order to fix a problem.
Adam Leventhal has written a very nice article on new Apple file system – APFS.