How well can you see in a particular situation?
That depends on three things:
bright light dim light darkness
Areas of dim light are considered lightly obscured; you have disadvantage on Wisdom (Perception) checks that rely on sight. Areas of darkness are considered heavily obscured; you effectively suffer the blinded condition when trying to see something in that area.
Range of light might be important. A torch provides bright light for a 20-foot radius, dim light for another 20 feet; beyond that you face darkness.
3. Physical Obstructions. Patchy fog or moderate foliage, even if bright light is present, will create a lightly obscured area. Here you have disadvantage on Wisdom (Perception) checks that rely on sight.
Opaque fog or dense foliage will create a heavily obscured area. Here your vision is entirely blocked and you effectively suffer the blinded condition.
You can't see something that has total cover. Your place in marching order may also result in your vision being lightly or heavily obscured.