Attached Link: http://www.debevec.org/

In a nutshell, it stands for High Dynamic Range Images. They are images that contain more information than the usual 24 bits we are used to. HDR images can be used to illuminate a scene, using just the colour and brightness values in the image. This means you can realistically light a scene with no lights. There's more to it than that, but that's the bare bones basics of it. The link here is a good place to start. I had another good one, but the site seems to be down.