Why An interlock is a safety switch wired into the cabinet or enclosure so that opening the case breaks the primary power circuit. Power supplies and amplifiers can hold hundreds or thousands of volts on filter capacitors and plate circuits, so the interlock removes the source of that voltage the moment the cover comes off. It protects the operator from contacting lethal potentials during service. Note that large filter capacitors can still hold a charge after power is removed, so bleeder resistors and a shorting stick are still good practice.
Watch out Shutting down on overheating describes a thermal cutout or thermostat, and shutting down on excess voltage describes an overvoltage crowbar circuit. Neither responds to the cabinet being opened.
Interlock = "open the lid, kill the volts." It senses the cabinet, not heat or voltage.