Updated June 10th, 2019.
If your Mac starts talking out loud, telling you which menu you’ve clicked on and which button is under your mouse, maybe the cat sat on the keyboard and pressed ⌘-F5. Try it yourself: press ⌘-F5 once to toggle it on, and do it again to toggle it off. For some reason, cats seem to be able to press those keys pretty easily.
Apple built the feature into the Mac system to help people who can’t see the screen. It’s really great, except when it takes you by surprise. Now you won’t be surprised.