The windows key opens the activities overview. You can usually find it next to the Alt key on your keyboard.
GNOME Documentation Project
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What is the "windows" key?
When you press the "windows" key, the activities overview is displayed. This key can usually be found on the bottom-left of your keyboard, next to the Alt key, and often has a window/squares icon on it. It is sometimes called the "super" key.
If you have an Apple Mac, there will not be a windows key on your keyboard. The Command (Cmnd) key can be used instead.
Need to add info on changing the keybinding using the new preferences windows.