Considering only black and white, levels define black as 0, and white as as the maximum in the collr space, say 255 in 8-bit color space.
Images do not always have that range. The lightest may not be true white,but a light shade of grey. When the "white" level is adjusted to lower than 255, such as in an image which might be no more intense than 216, everything gets interpolated upwards.
The reverse is that black adjustment for levels shifts everything darker.
And, hi! To me, image processing is very complicated. Luminosity, saturation, chromatic aberrations, hue equalization.. yikes!
