Sight

The most fundamental tasks of adventuring -- noticing danger, finding hidden objects, hitting an enemy in combat, and targeting a spell, to name just a few -- rely heavily on a character's ability to see. Darkness and other effects that obscure vision can prove a significant hindrance.
When targeting a creature you can see but who can't see you, you have Advantage.
Creatures, Objects, and sensory effects can be seen at different distances depending on their Size and assuming a clear line of sight:

• Tiny: 60 feet
• Small: 150 feet
• Medium: 300 feet
• Large: 500 feet
• Huge: 1000 feet
• Gargantuan: 2000+ feet

A given area might be lightly or heavily obscured:

• Lightly Obscured
• Heavily obscured

The presence or absence of light in an environment creates three categories of illumination: bright light, dim light, and darkness.

• Bright light
• Dim light
• Darkness

Certain creatures have special types of sight and perception:

• Blindsight
• Darkvision
• Truesight