Linear light is when you do the math right to represent how light interacts with the environment in terms of transmission, reflection, etc.

Most GFX tools use some kind of color space with a curve such as

https://en.wikipedia.org/wiki/SRGB

This is a practical answer to the problems of 8-bit technology where you would have terrible posterization in some values and also wasted bits in other values if you used linear light. sRGB is reflective of how the human eye works but not the physics of light. If you want to, say, average or subtract two images or rescale an image you should do it in linear light.

The right thing to do is to do transformations with linear light with at least 16 bit depth but output whatever color space the display expects. Most people (like the creators of Photoshop) do calculations that would be correct in linear light in sRGB and are oblivious to the results being wrong. It drives me nuts because my photog friends think I am nuts to write my own (correct) Python code with Pillow meantime they process everything with tools like

http://stereo.jpn.org/eng/stphmkr/

which makes everything look dark because the developers just didn’t think about color spaces.