Every body above absolute zero emits infrared radiation, and the amount depends on its temperature. A thermal camera measures that radiation pixel by pixel and translates it into an image where each colour corresponds to a temperature.
In electrical maintenance its great advantage is that it is done with the installation in service, at a distance and without contact: one round detects loose connections, load imbalance, degraded fuses and overheating motors without shutting anything down. ISO 18436-7 is the reference for personnel qualification, and the NETA specifications set the severity criteria.
Its two limitations must be kept in mind. The camera measures radiation, not temperature, so it needs the right emissivity: on bare shiny metal the reading is not reliable and the work is done by comparison between phases. And it only sees surfaces: a fault inside a closed enclosure does not show until it heats the metalwork.