Lightly Obscured

A Lightly Obscured space is one where sight is partly hindered, such as an area of Dim Light, patchy fog, or moderate foliage.

You have Disadvantage on Wisdom (Perception) checks that rely on sight to see something in a Lightly Obscured space.
Only sight-based perception is affected. Checks relying on other senses (hearing, smell) are unaffected.
Attack rolls are not affected. Being in or looking into a Lightly Obscured space does not impose Disadvantage on attacks, nor does it grant any cover or Advantage to attackers. (Contrast with a Heavily Obscured area, where you have the Blinded condition when trying to see something in it.)

Common source: An area of Dim Light is Lightly Obscured. Dim Light forms a boundary between Bright Light and Darkness, such as the outer edge of a torch's light, twilight, or a bright moon.