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When in the UV/Image Editor, left clicking on a pixel makes a small row of numbers appear at the bottom.

I know what some of these mean, but others are a mystery to me...

Obviously, X & Y are the coordinates you clicked on, and then R, G, and B are the RGB values for that pixel (for an 8 bit-per-pixel image, the value is converted from the 0-255 range to 0-1 range. However, the values can be much larger than that for other image types, such as .hdr or .exr. In fact they can be infinitely large). A is the Alpha (transparency) value of the pixel.

The numbers after "CM" are the Color-Managed values. Based on how you have set the Color Management section of your scene, it's telling you what the effective colors will be after applying the settings.

R, G, and B are adjusted for your color management, and the little square next to them is showing a visual representation of the adjusted color. Then H, S, and V are Hue, Saturation, and Value, which is simply a different way of representing the R, G, B values.

HSV is closer to how humans perceive color, whereas RGB is based on colors of light, so some people prefer to work in that color space.

The L is likely Luma, which is a gamma-corrected sum of R, G, and B. Basically if you converted a color to grayscale, how bright it would be. Since yours already is grayscale, the L value is the same as the RGB and V values.

Here's way too much detail: https://en.wikipedia.org/wiki/HSL_and_HSV

The RGB on the left are "scene-referred" values, meaning they are the actual values in the scene. The ones to the right of the "CM" indicator are "display-referred" values, meaning they are adjusted to show on a monitor based on your CM settings.

These are my scene settings, for example. If you change them you'll see the "CM" numbers will be different.

If you're not familiar with color management, I suggest watching this video: https://www.blenderguru.com/tutorials/secret-ingredient-photorealism