https://www.techradar.com/news/latest-windows-10-preview-jazzes-up-sign-in-screen-with-acrylic-effect

Microsoft is pretty much done with the Windows 10 October 2018 Update – save for some minor tinkering – but work continues apace on the following update, due in the first half of next year, with a fresh preview build emerging that brings the Fluent Design interface to the operating system’s sign-in screen.

To be specific, build 18237, which has been pushed out to ‘skip ahead’ testers, adds the acrylic effect to the background of said sign-in screen. Essentially, this means the background is blurred, to put the focus on the foreground and the box you use to actually log in, which makes sense.