While it’s surely the most important part of the GNOME Shell I left it at the last to give it more attention as it needs. I’m obviously talking about the Activities button, located in the upper left corner of the panel.
You can click upon it but you can also simply move the mouse pointer in the upper left corner of the screen to make it react as it was effectively pressed. In such case, a light blue animation (called ripple) is shown to help the user understand why the Activities screen was shown.
With the default keyboard configuration, the left Win key also works like to press the Activities button.