So, what you probably did to make it happen in the first place was hold down the Ctrl key while moving the thumbwheel on your mouse or the edge of the trackpad on your laptop. This is a shortcut to change the font size on your browser (you can also do this using the menus on your browser - how to do it varies according to the browser).
Ctrl-0 is a shortcut to restore the font size to 100%, whether you've zoomed in or zoomed out. Again you can do this using the menus, but Ctrl-0 is a lot quicker (and easier to describe).