This extension is pretty simple. If you start typing and the cursor is not inside a text box, it will show a red box at the bottom of the screen to try to catch your attention and let you know that what you are typing is not going anywhere.

To temporarily prevent the red box from showing on the current website, press the Escape key once.

You can turn off the sound in the settings page.

This extension does not record what you type, does not save anything and does not transmit anything off your computer.