Explain Like I'm 5 (ELI5)

No worries, color is complicated and fascinating!

"Brightness", in my experience, generally refers to the amount of light reflected (or sent from the light source). It doesn't directly affect the perceived hue, it's just a measure of how much light there is. It's hard to get an intuitive sense of brightness, because of how we perceive color.

Saturation refers to the "purity" of the color--that is, how much light it reflects of other wavelengths. Gray reflects visible light in equal proportions, which makes it the least saturated. A surface that reflects only a single wavelength would be considered fully saturated, and appear as a vibrant hue.

It might be more helpful to think about how our eyes receive different wavelengths. There are three colors we perceive: red, green, and blue. The closer the reflected wavelength is to these colors, the more those "sensors" in our eyes are activated. So for example, yellow activates the red and green sensors about the same amount (and less than pure red or pure green), so we perceive yellow. Orange is mostly the red sensors, with a little bit of green activated as well.

We perceive brown when the blue sensor is also activated by some amount, because brown reflects a little bit of blue light (as opposed to "pure" orange, which wouldn't have any blue light reflected). The lightness/darkness of the color is mostly the overall amount of light reflected (the brightness), but the big difference in what we perceive is that little extra blue light that brown reflects, that orange doesn't.

Brown is still considered "dark orange" because the primary wavelength that it reflects is an orange color. And dark browns don't need to reflect any blue light to be considered brown. But lighter brown shades will reflect a greater proportion of blue light than "light orange" would at the same brightness.