The dynamic range of an image is its full range of brightness from dark to light. The iPhone camera can balance the shadows with highlights to create a more dynamic image.
HDR photography is great for when your shot features the sky, or if an individual or object is heavily backlit, or if the sun is shining directly onto the subject.
The feature should be enabled by default on your device. To check, launch Settings and select Camera. You should see that Auto HDR is switched on. If you disable Auto HDR, you have the option of enabling it on a photo-by-photo basis — a HDR icon will appear at the top of the Camera app and you’ll be able to toggle it on or off.
In the same section of Settings, you can also choose whether you’d like to Keep Normal Photo. This will see your device retain two copies of each photo it captures — the HDR version and an original, unedited version. It’s true that sometimes, HDR photos don’t quite work. Keeping the original might be a good idea for serious iPhone-photographers.