light and vision

a given area might be lightly, heavily, or totally obscured

lightly obscured areas cause creatures to have disadvantage on wisdom (perception) checks that rely on sight

heavily obscured areas cause creatures to have disadvantage on attack rolls and on wisdom (perception) checks that rely on sight when the target of their attack, or whatever they are trying to perceive is heavily obscured

totally obscured areas cause a creature to effectively suffer from the blinded condition when trying to see something in that area

the presence or absence of light in an environment creates five categories of illumination: sunlight, bright light, dim light, darkness, and total darkness and magic can create a sixth, magical darkness

sunlight lets most creatures see normally, creatures with sunlight sensitivity have disadvantage on attack rolls and on wisdom (perception) checks that rely on sight when they, the target of their attack, or whatever they are trying to perceive is in direct sunlight

bright light lets most creatures see normally, bright light is the default condition it lit areas

dim light, creates a lightly obscured area, dim light can exist because an area is dimly light or is in an area or time between bright light and darkness

darkness creates a heavily obscured area, darkness happens when the barest of light still exists, like on moonless or overcast nights

total darkness creates a totally obscured area, total darkness happens when no light exists, this normally only occurs underground, inside structures, or it deep forests at night

magical darkness creates a totally obscured area even for those with darkvision, as the name implies, this only happens because of magic

blindsight

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

darkvision

many creatures, 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 that darkness, only shades of gray, it also treats areas of total darkness as darkness and so anything in it is heavily obscured

truesight

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 perceives the original form of a shapechanger or a creature that is transformed by magic, furthermore, the creature can see into the ethereal plane.