When Apple introduces macOS High Sierra in the fall, it will introduce a new file system to the Mac. The new operating system will phase out the HFS+ file system which the Mac has been using since Mac OS 8 (which was introduced in 1997) and will replace it with the new Apple File System (APFS) which is already being used in iOS 10.3.
This new operating system introduces an advanced 64-bit architecture and a responsive design to cut down on the time it takes to do common tasks. It also brings some heavy-duty security, including built‑in encryption, crash‑safe protections, and simplified data backup on the go. Copying files and backing up your data are two areas which will be dramatically faster. Overall, it should make your Mac noticeably snappier, and make it even more secure.
Under the hood, this is a big change, and while Apple is trying to make the switch as problem-free as possible, you will still want to take every precaution possible during the upgrade process when High Sierra becomes available in the fall.