According to the Blender manual, "Filmic" remaps the color space of your calculated image so that it fits into the color space that a monitor can display:
"There is also an artistic choice to be made. Partially that is because display devices cannot display the full spectrum of colors and only have limited brightness, so we can squeeze the colors to fit in the gamut of the device. Besides that, it can also be useful to give the renders a particular look, e.g. as if they have been printed on real camera film. The default Filmic transform does this. "
(source: https://docs.blender.org/manual/en/latest/render/color_management.html?highlight=filmic)