sRGB colorspace is a complex subject that involves understanding how monitors display color, how pictures/movies are stored, how our eyes percieve color and brightness and also how the limited precision of 8-bit images is optimized to store color information more efficiently. sRGB colorspace is very similar to a gamma 2.2 curve, with a linear portion at the low end.
Some articles that help explain sRGB:
Image files such as .jpg, .gif etc usually store the data in 8-bits per color channel. 8-bits per color channel only allows for 256 unique levels of information. This isn't enough information the accurately represent colors to the human eye, so tradeoffs need to be made. Human have better color perception for darks that brights, so more of these levels are used to encode the dark information than the light information. This is generally done by brightening the images with a gamma or sRGB curve. This brings up the dark colors and flattens out the bright ones, giving more levels to the darks.
Generally monitors don't display pixel values linearly. There is usually a curve to how much the brightness changes vs. the pixel's color. Many monitors use the inverse sRGB curve as their outputs, but calibration and other color adjustments affect this. This curve by default will make things darker, as is shown in the curve images in the listed articles. This is done so counter sRGB curve that is applied to images which the monitor manufactrers know they are usually showing. The two curves essentially cancel each other out, resulting in a visually linear display of brightness.
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