Exposure, Highlights, Shadows, Brightness, Contrast

• Highlights makes the bright things darker or brighter.
• Shadows makes then dark things darker or brighter
• Brightness makes everything brighter or darker.
• Contrast makes the bright things a bit darker and the dark things a bit brighter, or vice versa. Contrast either spreads out the range of brightnesses, or it reduces the range of brightnesses. In Photos, the Contrast adjustment seems to me to mostly adjust the dark end.

Really, you should try the adjustments and watch the Histogram and see what they do.

Here's a Histogram for a Monarch Butterfly:

Black is on the left and White is on the right. Most of the image is middle brightness.

Here's the Histogram after reducing the Highlights. There are no longer any Whites. The darks look about the same,

but the brightest part is gone, shoved over to be darker.

Shadows does the same thing, but on the left with dark parts.

Here I've increased the Brightness

so the center is more toward the Whites.

You should also watch the Luminosity Curve.

Different image editors shift the curve in somewhat different ways, with different cutoffs. In Photos, Highlights is the only one of the "Light" adjustments that does much to the brights. Contrast, Blacks, and Shadows all affect the darks in slightly different ways.

Brilliance adjusts contrast in a different way. It seems to be similar to what Lightroom calls Clarity-- all pretty fuzzy things.

Exposure in Lightroom seems to be the ISO or effective sensitivity. But in Photos it seems a bit flakey. and I don't find it very useful.

You can also move the 5 level adjustments directly on the Histogram for different cutoffs, which gives a great deal of flexibility.