The natural flux of ionizing radiation - both from cosmic radiation as well as natural radioactivity - can put crystalline structures in excited states storing some of the energy of the radiation. A very small fraction can stay in these excited states for long time. When such material is heated the stored energy is released as weak light, the so called thermoluminescence. After cooling and re-heating it will not show the light anymore, as no excited states remain.
Thermoluminescence dating is used for material where radiocarbon dating is not available, like sediments. Its use is now common in the authentication of old ceramic wares, for which it gives the approximate date of the last firing.