1st off, this upcoming change to Windows 10 will have the greatest impact on devices without much storage space, so if the PC, laptop, tablet etc. you're using has loads of free hard drive space, not to worry.
What's changing: starting with the next version upgrade, when you install Windows 10 fresh, approximately 7GB of hard drive will be reserved by the NTFS file system for Windows internal use only. IOW you lose that amount of hard drive space, and so far it doesn't look like there's anything that you can do about it. By the time it arrives however, there might be some exceptions -- what happens for example with those devices that shipped with only 16GB of storage space.
Why talk about it now, when the next upgrade is months off: it's in the news, so the odds are greater that you'll see it mentioned [not always accurately].
Why talk about backing up: Microsoft says [so far anyway] that this reserved chunk of storage space only exists when you buy a new device with Windows 10, or install a copy of Windows 10 fresh -- not upgrade versions. So in a nutshell, if/when losing 7GB of storage matters, you want to do everything you can to avoid installing Windows 10. Having a good backup of your current, working copy of Windows 10, minimizes the chances that you'll have to start over, reinstalling the OS. What would happen if you reset Windows 10 is not yet clear.
Why is this change happening: only my guess of course, but IMHO Microsoft as a whole is having identity issues -- despite the software providing a very large, crucial part of their income, Microsoft no longer wants to be the Windows company. They want to ship their own version of Android -- something that works on all sorts of portable devices and is simple enough that anyone can & will use it. But, they can't escape Windows software -- why else use Windows -- so they try instead to make the Windows user experience as similar to Android as possible. Their attempt at a touch screen interface failed, so they're concentrating on the update/upgrade process. They believe that having a guaranteed 7GB of space to store the downloaded files prior to upgrade will improve that user experience.
blogs[.]technet[.]microsoft[.]com/filecab/2019/01/07/windows-10-and-reserved-storage/