When human eyes glow in the dark it means that they are reflecting the light in a way that may appear as a glow.
The human eyes appear to glow in the dark due to the reflection of light off the retina, which is similar to the red eye effect that is seen in photos.
Humans lack the reflective layer called tapetum lucidum, like some nocturnal animals do, so the light that is reflected is weaker and also usually reddish as a result of the blood vessels in the retina.
In low light conditions, human eye pupils, dilate to allow more light to enter the eye.
And when a bright source of light such as a camera flash, is directed at your eye, the light reflects off of your retina and back out and causes a red reflection.
Human eyes do glow in a night vision camera although the human eyes glowing in a night vision camera is not a true glow of the eyes.
While human eyes do appear to glow in a night vision camera, the eyes glowing in the night vision camera is not in the same way that animals with a tapetum lucidum glow.
In night vision the human eyes glowing is due to the reflection of the infrared light which is often from the infrared illuminator's on the night vision camera which reflects off the choroid, which is a layer of blood vessels that are behind the eyes retina.
Human eyes lack the reflective tapetum lucidum that is found in many types of nocturnal animals, but human eyes still reflect some light and most particularly when you use IR or infrared illumination.
Night vision cameras often use either active IR or infrared illumination or passive IR infrared illumination to see in the dark.
And many types of night vision cameras also have infrared LEDs which emit invisible light to illuminate the scene.
The infrared light is reflected off of objects, which also include the human eye and the light then also reflects off of the choroid which is the layer that is behind your retina, that causes a visible reflection, which in turn appears as a glow in the night vision cameras glow.
The effect of the human eye glowing in a night vision camera is different from the eyeshine which is seen in nocturnal animals like deer or cats that have tapetum lucidum which reflects much more light.
The reflection from our human eyes is much weaker than the reflection of the nocturnal animals eyes due to us humans lacking the tapetum lucidum.
The red eye effect in a regular flash camera is also a result of the reflection off of the blood vessels in the choroid.