HDR stands for High Dynamic Range.
In photography, a setting with a high dynamic range is one where there is a very wide range of lights to darks. For instance if you are in a home with large windows on a bright sunny day, there is a huge difference between the light coming in from the windows compared to the ambient light given off by fixtures in the home. Our eyes and mind do a terrific job adapting and balancing that range of light. We usually can still see the detail in the room even though there is bright light coming in from the windows. However the sensor on a camera, even some of the most sophisticated cameras in the world, still have a hard time doing what our brain and eyes do. How many pictures in real estate listings have you seen where it looks like a nuclear blast is occurring through the window; or the outside looks fine and the room is almost totally black?
This is where HDR photography can be used effectively to give a good view of the interior and exterior of a home.
The process of HDR involves taking multiple exposures that cover the entire gamut of light and then blending those images in a specific HDR blending software. Today we have cameras that can do this automatically but I am not aware of one that produces clean, good quality images directly out of camera.
As you can see from the image above, a prospective buyer can see the view from the doors instead of just bright daylight.