macOS High Sierra drops today. Under the hood, this is the most ambitious major operating system upgrade since, at least, Windows 2000/XP and MacOS X 10.0, which is to say, in the last 20 years. Apple is attempting to do an in-place file system upgrade on all modern Macs (Macs with solid states drives), replacing the ancient HFS+ with the new APFS. This sort of thing hasn’t been attempted in modern times on PCs (Apple successfully tested the process earlier this year on iOS and its progeny--tvOS and watchOS--but those are far more closed, controlled environments).
Microsoft gave it a shot during the Vista development cycle, but abandoned the attempt when the logistics became too complicated. Even today, MS has a modern file system in the wings, ReFS, but has not implemented it, except in voluntary situations on Server 2012 R2 and 2016. They are frightened, as they should be. A lot can break. Interestingly, one of the things ReFS doesn’t support, which MS badly needs in Hyper-V, is a file system that supports access from multiple hosts at once, like VMware’s VMFS.
But back to Apple. Be careful. There’s no need to upgrade to High Sierra today. Wait a month or two. Apple isn’t making a big deal about APFS, but the change under the hood is radical and unprecedented. Things WILL break. If you do make the switch, be sure every single application you run has been updated to support APFS and that you have a solid backup of your entire system, pre-upgrade.