>>714965115HDR is basically just extra brightness headroom that your monitor has to make things like bright lights and specular highlights actually, physically bright, instead of them being limited to boring, dim "screen" white like what you'd see in a text document.
It also tends to make images slightly more vibrant, by virtue of bright scene-data not needing to tone-map to a white roll-off. And there is additional headroom for more vibrant colors, which could be useful for certain scenes like neon cityscapes or places with artificial light sources.