A low-emissivity coating is a metallic oxide layer measured in nanometers, deposited on one of the interior surfaces of an insulated unit. It is essentially transparent to visible light but highly reflective to long-wave infrared — the wavelength at which warm objects, including your radiators and your body, radiate heat.
In winter, that means interior heat trying to escape through the glass is reflected back into the room. In summer, solar infrared arriving from outside is reflected away before it becomes cooling load. A single well-specified coating typically cuts radiative heat transfer through the glazing by roughly half.
Coating placement matters. Surface two (the inside face of the outer lite) favors solar rejection, which suits cooling-dominated climates and west-facing elevations. Surface three favors heat retention, which suits heating-dominated climates. The same glass package specified for the wrong surface produces measurably different comfort.
Modern coatings are also tuned for spectral selectivity — high visible transmittance paired with low solar heat gain. This is what allows a restored window to gain performance without acquiring the grey tint that early tinted glazing imposed on interiors.