It’s the November update, aka Windows 10 1511 (15 – 2015, 11 – November)
These will occur at regular intervals (half yearly, maybe quarterly). Windows 10 is a change from the past. Instead of releasing a new version of Windows after a long delay of many years and not getting any new features in the meantime, there will be no Windows 11, 12, etc. Just Windows 10 but with feature updates on a more regular basis.
These are actually full installs of Windows so may reset a few things but shouldn’t trash everything if it runs as an update. Just maybe annoying if it resets some settings. It can also install new drivers in the process which might mess things up if you’ve been using a driver from a manufacturer web site because it’s better or fixes the official driver in Windows Update.
You can also download the update as an ISO and that’s then the only disc you need if you want to install it fresh.
It’s basically taking the approach Linux distributions take, except that the big updates are forced on people to try to avoid issues of fragmentation and having to support lots of different versions.
The downside of this approach is a lot of features are beta quality. They are being tested on the ‘insider’ program (which you can opt into if you want more frequent updates), but still seem to be pushed out for release incomplete and with bugs.