When the sun shines, very high contrast is created between highlight and shadow areas of a scene. Dark shadows without detail, people with squinting eyes, and very bright highlights are trademarks of a high contrast day.

For some reason, we are under the impression that sunny days are the best days for photography. ironically, overcast days are better for photographing people.

Light rays are scattered, or diffused, on overcast days. Dark shadows don't exist. We use terms like "low contrast" or "flat" to describe this lighting that is so flattering to photos of people. The light is "softer" than a normal contrast day.

Specular highlights (glare) doesn't exist pn flat days. Photos will lack "sparkle" .

Supplemental lighting can be added to increase highlights in low contrast situations.