Undo use of F5 which added a thick black line around fields and buttons I have searched the user manuals and can find no information on using F keys and how to cancel a command I have done.
macOS 10.8.5

I foolishly followed some advice and used F5 button for something. Now there is a permanent open field on all windows and Excel won't let me input into individual fields. 

I don't remember what I did, why I did it, and don't know how to undo it. Thick black line flashes round a field box, or any button.

How can I undo this action?
You appear to have enabled the Accessibility shortcut.

To toggle it back off, option+command+F5