What is "day/night" feature of security camera?

Answer 1

Many cameras have a day/night feature which adjusts the aperture (the lens opening) so that at night, more light can be captured. In the day, the aperture is closed down so that the pictures are not washed out by too much light.

Answer 2

The ability to "see" in the dark using infrared technology (and perhaps an infrared light source to supplement) as well as seeing in the daylight hours.