HDR is High Dynamic Range, its something to do with lighting. It makes the lightning much more dynamic as in one person explained it to me, it makes the lights lighter and the darks darker. Things like the gleam on a sword or shadows atmosphere and so on. Never saw it personally so I cant honestly comment on it but from those who have seen it, it is apparently pretty amazing. Especially when you can combine it with FSAA (Full Screen Anti Aliasing). FSAA helps to eliminate Jaggies, very very nice in certain games.
Hope the nfo helps friend.