Night vision is used to to enhance sight in the dark. Most instances use a combination of two approaches: Enhanced spectral range, and enhanced intensity range. Enhanced spectral range is where the viewer is sensitive to light beyond what would be considered normal by a human observer. Human vision is confined to a very small portion of the electromagnetic spectrum which is considered light. This allows the viewer to take advantage of additional sources of light, or make more effective use of the available sources of light.
Enhanced intensity range is simply the ability to see with very small quantities of light. Although the human visual system can, in theory, detect single photons under ideal conditions the neurological noise filters limit sensitivity to where a few tens of photons are required even in ideal conditions Some animals have achieved improved night vision evolutionarily though the use of a larger optical aperture, improved retina composition which can detect weaker light over a larger spectral range, more photoefficent optics in the eye, and improved neurological filtering which is more tolerant of noise.
In biological night vision, rhodopsin in the rods of the eye breaks as light hits it. The peak rhodopsin build up time for optimal night vision in humans is 30 minutes. Rhodopsin in the human rods isn't sensitive to the longer red wavelengths of light, so many people use red light to preserve night vision as it will not deplete the eye's rhodopsin stores in the rods and instead is viewed by the cones.