High frequency radio waves are transmitted over a given surface. It could be a secure enclosure or the surface of a laptop.
The radio waves move over the surface but do not transmit away from it. Like ripples on a pond. If something on the surface blocks them, or the surface changes, then the signal is altered.
The shape of the received radio signal is a fingerprint of the surface. This can be used to monitor its integrity and to check it has not been touched, damaged or replaced.