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 (see the rules glossary) when trying to see something there.