Darkvision referred to the ability to see in the dark by allowing a creature to discern forms, as in normal daylight vision, only in shades of greys. Monsters with this ability could only see in the dark up to a certain radius.
It was similar to low-light vision, a trait more commonly found among surface races. Underground creatures often had darkvision, including drow, illithids, and minotaurs. Many outsiders also possessed darkvision.