[PLUG-TALK] What the heck is a super key?

[John Jason Jordan]

I just did a dist-upgrade on my Lenovo laptop to Xubuntu 20.04.1, and
now I'm fixing stuff that broke. In the process suddenly my Chromium
browser went to full screen and I couldn't get it back to unmaximized.
There was no title bar, no icons in the corners; it had taken over the
entire display. Finally I succeeded in getting the panel visible, where
I launched Firefox to search for a solution. All I could find was that
I should hold down the super key while clicking on the title bar. Never
mind that I had no visible title bar, I also had no super key. But it
might have been at least partially useful if I knew what a super key
was.

1) What might I have accidentally done to make Chromium go to full
screen (I never run any programs full screen), and why did it have no
title bar, icons, or window borders?

2) If this ever happens again, what is a keyboard shortcut to
unmaximize a window, hopefully not requiring a super key?

3) What the heck is a super key?