The most fundamental tasks of adventuring---noticing danger, finding hidden objects, hitting an enemy in combat, and targeting a spell, toname just a few---rely heavily on a character's ability to see. Darknessand other effects that obscure vision can prove a significant hindrance.
A given area might be lightly or heavily obscured. In a lightly obscured area, such as dim light, patchy fog, or moderate foliage,creatures have disadvantage on Wisdom (Perception) checks that rely onsight.
A heavily obscured area---such as darkness, opaque fog, or densefoliage---blocks vision entirely. A creature effectively suffers fromthe blinded condition when trying to see something in that area.
The presence or absence of light in an environment creates threecategories of illumination: bright light, dim light, and darkness.
Bright light lets most creatures see normally. Even gloomy daysprovide bright light, as do torches, lanterns, fires, and other sourcesof illumination within a specific radius.
Dim light, also called shadows, creates a lightly obscured area. Anarea of dim light is usually a boundary between a source of brightlight, such as a torch, and surrounding darkness. The soft light oftwilight and dawn also counts as dim light. A particularly brilliantfull moon might bathe the land in dim light.
Darkness creates a heavily obscured area. Characters face darknessoutdoors at night (even most moonlit nights), within the confines of an unlit dungeon or a subterranean vault, or in an area of magical darkness.