Vision and Light

Some adventuring tasks-such as noticing danger, hitting an enemy, and targeting certain spells-are affected by sight, so effects that obscure vision can hinder you, as explained below.
Obscured Areas
An area might be Lightly or Heavily Obscured. In a Lightly Obscured area-such as an area with Dim Light, patchy fog, or moderate foliage-you have Disadvantage on Wisdom (Perception) checks that rely on sight.
A Heavily Obscured area-such as an area with Darkness, heavy fog, or dense foliage-is opaque. You have the Blinded condition when trying to see something there.
Light
The presence or absence of light determines the category of illumination in an area, as defined below.

Special Senses
Some creatures have special senses that help them perceive things in certain situations. "Rules Glossary" defines the following special senses:
Blindsight
Darkvision
Tremorsense
Truesight