This is a stick-on cover for a standard North American household wall switch that prevents it from being toggled by accident.
With this cover attached you won't be able to flip the switch by brushing it with your shoulder, hitting it accidentally etc.
Useful where a wall switch controls sensitive equipment, such as a furnace or (as in my case) cuts power to the outdoor light replaced with a smart floodlight with integrated security camera (guests often flipped this switch at the front door thinking that it turns on the overhead light).
The cover has a hole in it so that you can flip the switch with your little finger easily if needed. Inside clearance is sufficient to accommodate switches that stick out a little (not flat with the faceplate) due to subpar installation.
Use some double-sided adhesive tape to attach this cover to a switch and enjoy )