Webcam cover are simple devices that physically cover the webcam on your phone or computer. This will prevent a hacker from spying on you — even if they are able to surreptitiously turn on your webcam.
I recommend these Webcam Covers ($8 for 5 covers). They are made out of metal instead of the usual plastic. They are still very thin (0.027 inches) so they do not prevent laptops from closing perfectly. They can be opened and closed at will.