TEMPEST shielding is the process of protecting sensitive equipment from emanating electromagnetic radiation (EMR) that may carry classified information. This is to prevent it from being intercepted by outside entities. It may also refer to the shielding itself, which is applied to electronic equipment to inhibit the transmission of EMR. This type of protection resulted as a precautionary measure for keeping secrets hidden during the height of political espionage during the Cold War.
Techopedia explains TEMPEST Shielding
TEMPEST is a code word for a classified U.S. government project and study into the susceptibility of computer systems and telecommunications equipment to spying by reconstructing intelligible data from the electromagnetic resonance they emit. TEMPEST was later considered as an acronym for Transient Electromagnetic Pulse Surveillance Technology, which refers to the equipment and devices that emit or receive and decipher data from electromagnetic resonance, referred to as compromising emanations.
TEMPEST shielding is therefore both the act and the material itself used to stop these electromagnetic emanations from reaching devices that may be used to decipher them. The process follows the "red/black" separation principle in which the "red" equipment, such as computer terminals which carry confidential data, must be isolated from the "black" equipment, such as radios and modems, which may capture signals through various filters and shields: the TEMPEST shielding. It is also often placed in whole rooms or even buildings to ensure that no EMR leaks out, possibly compromising security by inadvertently leaking data.