1. A technique for producing images that have good detail in both bright areas and dark areas of a scene with high contrast.
For example, with traditional photography, a daytime photo of a bridge against a sky might be exposed so that clouds can be seen clearly, but the shadows of the bridge would be black. Exposing the photo so that details in the shadows could be seen clearly would turn the sky white. HDR would create a photo where both the sky and the shadows under the bridge could be seen with good detail.
HDR typically works by taking 2-3 photos rapidly at different exposures (one over-exposed and one under-exposed) and then digitally combining them to merge the best of both into one image.
Some phones can also apply this technique to video.
2. A video format that defines at least a billion distinct colors, for greater contrast and detail, usually noticeable in the darkest and lightest parts of a scene.
See: HDR10