Vision & Light
PHB pg. 183
In a lightly obscured area creatures have disadvantage on Wisdom (Perception) checks that rely on sight.
In a heavily obscured are creatures effectively suffer from the blinded condition when trying to see something in that area.
Bright light lets most creatures see normally.
Dim light, also called shadows, creates a lightly obscured area.
Darkness creates a heavily obscured area.
A creature with blindsight can perceive its surroundings without relying on sight, within a specific radius. Creatures without eyes, such as oozes, and creatures with echolocation or heightened senses, such as bats and true dragons, have this sense.
Many creatures in the worlds of D&D, especially those that dwell underground, have darkvision. Within a specified range, a creature with darkvision can see in dim light as if it were bright light and in darkness as if it were dim light, so areas of darkness are only lightly obscured as far as that creature is concerned. However, the creature can’t discern color in darkness, only shades of gray.
A creature with truesight can, out to a specific range, see in normal and magical darkness, see invisible
creatures and objects, automatically detect visual illusions and succeed on saving throws against them,
and perceive the original form of a shapechanger or a creature that is transformed by magic. Furthermore, the creature can see into the Ethereal Plane.