A photographic negative. Every value flips: black becomes white, white becomes black, and each colour swaps for its opposite.
mode offers a second, stranger option called solarize. Instead of flipping everything, it leaves the dark half of the range alone and folds the bright half back down. The effect is that both pure black and pure white come out black, while the midtones stay bright. Tones that were far apart end up next to each other, which is why solarized images look metallic and unreal.
This is not an invention of image editors. Overexpose a piece of film part way through developing it and the highlights reverse in exactly this way, a mishap photographers noticed and then started doing on purpose. It carries the name of Armand Sabattier, who wrote it up in 1862.