TES is a spectrometer that measures the infrared-light energy (radiance) emitted by Earth's surface and by gases and particles in Earth's atmosphere. Every substance warmer than absolute zero emits infrared radiation at certain signature wavelengths. Spectrometers measure this radiation as a means of identifying the substances.
TES has very high spectral resolution, which gives it the ability to pinpoint the wavelengths at which the substances are emitting. This enables precise identification of the substances, and also provides information about their location in the atmosphere. Emission wavelengths can vary with temperature and pressure, so seeing the emissions with great precision enables scientists to infer the temperature and pressure of the chemicals from which they came. This, in turn, implies that the chemicals being observed are at a certain altitude where those temperatures and pressures apply. The ability to determine the altitude of the observed chemicals enables TES to distinguish radiation from the upper and lower atmosphere, and focus on the lower layer - the troposphere.
Since it observes light in the infrared range of the electromagnetic spectrum, similar to night-vision goggles, TES can observe both day and night. Its spectral range overlaps that of HIRDLS, another of the instruments aboard the Aura satellite. So, in addition to its work in the troposphere, TES can supplement HIRDLS measurements of chemicals in the stratosphere that are common to both instruments, as well as help scientists measure additional constituents of the stratosphere.