Ah! My bad - it's SHIFT+WIN+S, not CTRL+WIN+S! Sorry for the confusion.
SHIFT+WIN+S activates the "Snip & Sketch" tool and allows the user (on a Windows environment) to snap a section of the screen and then mark it up with some basic markup features. I used to use the "Snipping Tool" to achieve the same thing, but Snip & Sketch is better because of the ruler and keyboard shortcut features.
Snip & Sketch is still missing some features - for example you can't add text notes or basic shapes and the ruler rotation is a bit clunky. But it's pretty good for most situations, and you can still paste the snipped section into your image manipulation tool of choice.
Note: I'm using Windows 10 1903. I think Snip & Sketch was added only in the last couple of years, so older versions of Windows 10 might not have it.
Thanks for your vote on this item though