Stage Manager in Mac OS is not a secret, but I’ve only learned about it recently.
It’s off by default so you have to enable it in system settings:

It’s hard to describe in words, so you’ll have to try it. And experiment a bit because I didn’t get in the first hour.
It’s a certain way to manage windows for less clutter. Imagine you have a browser, an editor and a terminal i.e. 3 windows. Might be annoying to have them all shown on screen.
When Stage Manager is enabled, thumbnails of windows are on the left edge of the screen and you can switch to the window by clicking the thumbnail.
By default Stage Manager shows each window by itself on screen. You can group them by dragging thumbnail onto screen. For example, when I’m developing, I’m using text editor and terminal so I will group them so they are both visible on screen at the same time, but not the other apps.
So far I’m enjoying using Stage Manager.